Power Supplies Biasing BJT and MOSFET

Power Supplies Biasing BJT and MOSFET
UNIT I Power Supplies Biasing BJT and MOSFET

Outline Rectifier BJT Biasing FET Biasing

Block diagram of Power Supply Unit

RECTIFIER Transformer: To Step down AC voltage amplitude to the desired DC voltage (by selecting an appropriate turn ratio N1/N 2 for the transformer) Isolate equipment from power-line . Rectifier: Converts an ac input to a unipolar output Filter.

Filter Convert the pulsating input to a nearly constant dc output Regulator. Reduce the ripple of the dc voltage.

Half wave Rectifier

Half wave Rectifier- contd…

Half wave Rectifier -contd…
The input is an alternating current. This input voltage is stepped down using a transformer. The reduced voltage is fed to the diode ‘D’ and load resistance RL. During the positive half cycles of the input wave, the diode ‘D’ will be forward biased.

Half wave Rectifier-contd…
During the negative half cycles of input wave, the diode ‘D’ will be reverse biased. We take the output across load resistor RL.

The diode passes current only during one half cycle of the input wave. The output is positive and significant during the positive half cycles of input wave.

At the same time output is zero or insignificant during negative half cycles of input wave. This is called half wave rectification. Dc current given by

The ratio of dc power output to the applied input a.c power is known as rectifier efficiency, denoted by η.

Half wave rectifier with filter

Half wave rectifier with filter-contd…

Output of half wave rectifier is not a constant DC voltage. It is a pulsating dc voltage with ac ripples. In real life applications, we need a power supply with smooth wave forms. we desire a DC power supply with constant output voltage.

We can make the output of half wave rectifier smooth by using a filter (a capacitor filter or an inductor filter) across the diode. We can also use an resistor-capacitor coupled filter (RC).

Full wave rectifier

Full wave rectifier-contd…
In a Full Wave Rectifier circuit two diodes are used, one for each half of the cycle. A multiple winding transformer is used whose secondary winding is split equally into two halves with a common centre tapped connection, (C).

When point A of the transformer is positive with respect to point C, diode D1 conducts in the forward direction as indicated by the arrows. When point B is positive (in the negative half of the cycle) with respect to point C, diode D2 conducts in the forward direction. The current flowing through resistor R is in the same direction for both half-cycles.

This configuration results in each diode conducting in turn its anode terminal is positive with respect to the transformer centre point C producing an output during both half-cycles, twice that for the half wave rectifier so it is 100% efficient.

Unidirectional current given by

Bridge Rectifier

Bridge Rectifier-contd…
This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop “bridge” configuration to produce the desired output. The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost.

The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below. The four diodes labelled D1 to D4 are arranged in “series pairs” with only two diodes conducting current during each half cycle.

During positive cycle of the input

During the positive half cycle of the supply, diodes D1 and D2 conduct in series Diodes D3 and D4 are reverse biased and the current flows through the load as shown below.

During Negative cycle of the input

During the negative half cycle of the supply, diodes D3 and D4 conduct in series. Diodes D1 and D2switch “OFF” as they are now reverse biased. The current flowing through the load is the same direction as before.

The current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier. The average DC voltage across the load is 0.637Vmax.

The smoothing capacitor converts the full-wave rippled output of the rectifier into a smooth DC output voltage.

Bipolar Junction Transistor(BJT)
Bell Labs (1947): Bardeen, Brattain, and Shockley Originally made of germanium Current transistors made of doped silicon

The basic of electronic system nowadays is semiconductor device. The famous and commonly use of this device is BJTs (Bipolar Junction Transistors). It can be used as amplifier and logic switches

Point-Contact Transistor –
first transistor ever made

BJT consists of three terminal:  collector : C  base : B emitter : E Two types of BJT : pnp and npn 3 layer semiconductor device consisting: 2 n- and 1 p-type layers of material  npn transistor 2 p- and 1 n-type layers of material pnp transistor The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material

Basic models of BJT npn transistor Diode Diode pnp transistor Diode

Transistor currents The arrow is always drawn on the emitter
The arrow always point toward the n-type The arrow indicates the direction of the emitter current: pnp:E B npn: B E IC=the collector current IB= the base current IE= the emitter current

Understanding BJT working
force – voltage/current water flow – current - amplification

Transistor working PNP

Both biasing potentials have been applied to a pnp transistor and resulting majority and minority carrier flows indicated. Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type material. A very small number of carriers (+) will through n-type material to the base terminal. Resulting IB is typically in order of microamperes. The large number of majority carriers will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal.

Majority carriers can cross the reverse-biased junction because the injected majority carriers will appear as minority carriers in the n-type material. Applying KCL to the transistor : IE = IC + IB The comprises of two components – the majority and minority carriers IC = ICmajority + ICOminority ICO – IC current with emitter terminal open and is called leakage current.

Operation region summary
IB or VCE Char. BC and BE Junctions Mode Cutoff IB = Very small Reverse & Reverse Open Switch Saturation VCE = Small Forward & Forward Closed Switch Active Linear VCE = Moderate Reverse & Forward Linear Amplifier Break-down VCE = Large Beyond Limits Overload

Transistor Bias Circuits

Objectives Discuss the concept of dc biasing of a transistor for linear operation Analyze voltage-divider bias, base bias, and collector-feedback bias circuits. Basic troubleshooting for transistor bias circuits

Introduction For the transistor to properly operate it must be biased.
Several methods to establish the DC operating point. We will discuss some of the methods used for biasing transistors as well as troubleshooting methods used for transistor bias circuits.

Common Emitter Configuration
_ + Ie Ic Ib Vee Vcc Re Rc Rb1 Rb2 Vcc Input Output E C B Input Output The circuit has been re-configured with input at Base & output at Collector The Emitter is common to input & output This is called Common Emitter configuration

Input Characteristics

Output Characteristics

Beta () or amplification factor
The ratio of dc collector current (IC) to the dc base current (IB) is dc beta (dc ) which is dc current gain. where IC and IB are determined at a particular operating point, Q-point (quiescent point).

Common Base Configuration
- Vbe _ + E B Ie Ic Ib Vcb C Input Output Here the input is applied at the Emitter & the output taken from the Collector In this arrangement Base is common to the input & output This is called Common Base configuration

Common Base

In the dc mode the level of IC and IE due to the majority carriers are related by a quantity called alpha = IC = IE + ICBO It can then be summarize to IC = IE (ignore ICBO due to small value)

Alpha a common base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector. The value of  is typical from 0.9 ~

Common – Collector Configuration
Also called emitter-follower (EF). It is called common-emitter configuration since both the signal source and the load share the collector terminal as a common connection point.

Common – Collector Configuration
The output voltage is obtained at emitter terminal. The input characteristic of common-collector configuration is similar with common-emitter. configuration.

Common Collector characteristics

Output characteristics of CC configuration

Operating Regions Region of operation E – B junction C – B Cut off
Reverse Biased Active Forward Biased Saturation

Ic Active Region Cut-off Region Vce Saturation Region Ib = 60μA
Ic = 10mA Ib = 50μA Ic = 8mA Ib = 40μA Ic = 6mA Ib = 30μA Ic = 4mA Cut-off Region Ib = 20μA Ic = 2mA 0 V 24 V Vce

Transistor as an amplifier

Simulation of transistor as an amplifier

The DC Operating Point The goal of amplification in most cases is to increase the amplitude of an ac signal without altering it. Fig 5-1a, b, & c

The DC Operating Point For a transistor circuit to amplify it must be properly biased with dc voltages. The dc operating point between saturation and cutoff is called the Q-point. The goal is to set the Q-point such that that it does not go into saturation or cutoff when an a ac signal is applied.

Q-Point (Static Operation Point)
When a transistor does not have an ac input, it will have specific dc values of IC and VCE. These values correspond to a specific point on the dc load line. This point is called the Q-point. The letter Q corresponds to the word (Latent) quiescent, meaning at rest. A quiescent amplifier is one that has no ac signal applied and therefore has constant dc values of IC and VCE.

DC Biasing Circuits The ac operation of an amplifier depends on the initial dc values of IB, IC, and VCE. By varying IB around an initial dc value, IC and VCE are made to vary around their initial dc values. DC biasing is a static operation since it deals with setting a fixed (steady) level of current (through the device) with a desired fixed voltage drop across the device.

The DC Operating Point The goal is to set the Q-point such that that it does not go into saturation or cutoff when an a ac signal is applied. Fig 5-2a & Fig 5-4

Collector characteristic curves
The collector characteristic curves graphically show the relationship of collector current and VCE for different base currents.

The DC Operating Point With the dc load line superimposed across the collector curves for this particular transistor we see that 30 mA of collector current is best for maximum amplification, giving equal amount above and below the Q-point. Fig 5-2a & Fig 5-4

The DC Operating Point-contd…
Effect of a superimposed ac voltage has on the circuit. The collector current swings do not exceed the limits of operation(saturation and cutoff). Applying too much ac voltage to the base would result in driving the collector current into saturation or cutoff resulting in a distorted or clipped waveform. Fig 5-5 circuit and load line w/signals

Voltage swing Both AC and DC load lines are shown as drawn on the collector characteristics of an NPN transistor. Note that both of the lines have to pass through the operating point, Q.

Voltage swing-contd… AC load line defines the range the collector current and voltage swings that can take place around the operating point. The range limited on by the saturation region of the transistor characteristics and on the right by its cut-off point.

Voltage swing-contd… If the swings ten exceed these limits, the waveform is clipped, creating severe distortion in the amplified signal. The undisto (unclipped) voltage swing is restricted to ∆vMAX+ and ∆vMAX+ around the operating point

Voltage swing-contd…

Voltage-Divider Bias Voltage-divider bias is the most widely used type of bias circuit. voltage-divider bias is more stable( independent) than other bias types. Fig 5-9 Voltage-Div. Bias

Voltage-Divider Bias-contd…
R1 and R2 are used to provide the needed voltage to point A(base). The voltage at point A of the circuit in two ways, with or without the input resistance(point A to ground) considered. Fig 5-9 Voltage-Div. Bias

Fig 5-10a & b

The voltage across R2(VB) by the proportional method. VB = (R2/R1 + R2)VCC Fig 5-9 Voltage-Div. Bias

Base voltage and subtract VBE to find out what is dropped across RE ,determine the current in the collector-emitter side of the circuit. The current in the base-emitter circuit is much smaller, IE≈ IC Fig 5-9 Voltage-Div. Bias

Base Bias This type of circuit is very unstable since its  changes with temperature and collector current. Base biasing circuits are mainly limited to switching applications. Fig 5-19 Base bias circuit

Collector-Feedback Bias
Collector-feedback bias is kept stable with negative feedback, although it is not as stable as voltage-divider or emitter. With increases of IC, less voltage is applied to the base. With less IB ,IC comes down as well.. Fig 5-23 collector feedback IB = (VC - VBE)/RB IC = (VCC - VBE)/(RC + RB/DC)

Base Bias Vc = Vcc – (Ic + Ib) x Rc Also, Vc = (Ib x Rb) + Vbe
Vce Rc Rb Ic Vc = Vcc – (Ic + Ib) x Rc Also, Vc = (Ib x Rb) + Vbe Equating the two equations Vcc – (Ic + Ib)Rc = (Ib Rb) + Vbe Or, Ib(Rc + Rb) = Vcc – IcRc - Vbe Ib = Vcc – IcRc - Vbe Rc + Rb . . . Ic = ( Vcc – IcRc – Vbe) Rc + Rb As Ic = Ib

Disadvantages of fixed bias circuit
Ic increases with temperature & there is no control over it Hence there is poor thermal stability Ic =  Ib Hence Ic depends on   may change from transistor to transistor This will shift the operating point Hence stabilization is very poor in fixed bias circuit

Advantages of fixed bias circuit
Simple circuit with minimum components Operating point can be fixed conveniently in the active region, by selecting appropriate value for Rb Hence fixed bias circuit provides flexibility in the design

Emitter Bias This type of circuit is independent of  making it as stable as the voltage-divider type. The drawback is that it requires two power supplies. IB ≈ IE/ IC ≈ IE ≈( -VEE-VBE)/(RE + RB/DC) Fig 5-21a npn emitter bias

Summary The purpose of biasing is to establish a stable operating point (Q-point). The Q-point is the best point for operation of a transistor for a given collector current. The dc load line helps to establish the Q-point for a given collector current. The linear region of a transistor is the region of operation within saturation and cutoff.

Stability Factor

Stability Temperature & Current gain variation may change the Q point
Stability refers to the design that prevents any change in the Q point Temperature effect When the temperature increases it results in the production of more charge carriers This increases the forward bias of the transistor and Ib increases

Temperature effect When the temperature increases it results in the production of more charge carriers This increases the minority charge carrier and hence the leakage current as Iceo = (+1) Icbo Icbo doubles for every 100 C As Ic =  Ib + Icbo The increase in the temperature increases Ic This in turn increases the power dissipation and again more heat is produced

Stability Factor It indicates the degree of change in the operating point due to variation in temperature There are 3 stability factors corresponding to the 3 variables – Ico, Vbe &  S Ic Ico = Vbe,  constant The stability factor should be as minimum as possible S’ Ic Vbe = Ico,  constant S’’ Ic  = Ico, Vbe constant

Techniques Stabilization technique
Resistive biasing circuits change Ib suitably and keep Ic constant Compensation technique Temperature sensitive devices such as diodes, thermistors & transistors are used to provide suitable compensation and retain the operating point without shifting

Stability Factor S For Fixed Bias Circuit Ic Ico S = = ( I +  )
Vbe,  constant S = = ( I +  ) 1 -  Ib Ic For the fixed Bias Circuit Ib = Vcc / Rb Ib Ic . . . = 0 S = ( I +  ) 1 - (0) . . . S = 1 +  . . .

Stability Factor S’ S’ = For Fixed Bias Circuit Ic =  Ib + Iceo Ic
Vbe = Ico,  constant =  Ib + ( + 1) Icbo =  Vcc - Vbe Rb + ( + 1) Icbo = - + ( + 1) Icbo Rb  Vcc  Vbe . . . + Ib Vbe = Rb  _ S = -  / Rb . . .

Stability Factor S’’ For Fixed Bias Circuit Ic = Ib + Iceo S’’   =
Ico, Vbe constant = Ib + (+1)Icbo =  Vcc - Vbe Rb + ( + 1) Icbo = - + ( + 1) Icbo Rb  Vcc  Vbe . . . Ic  = - + Icbo Rb Vcc Vbe = Ib + Icbo = Ib (approx) = Ic /  S’’ = Ic /  . . .

For Collector-Base Bias
Stability Factor S’ For Collector-Base Bias S’ Ic Vbe = Ico,  constant Ib = Vcc – IcRc - Vbe Rc + Rb = Vcc – IcRc - Vbe Rc + Rb Ic  Ic = Rb + ( + 1) Rc (Vcc – Vbe)  Ic + Rc + Rb IcRc Vcc - Vbe = S’ Ic Vbe = Rc + Rb + Rc Ic  (Rc + Rb) Rc + Rb Vcc - Vbe = = Rb + ( + 1) Rc - 

For Collector-Base Bias
Stability Factor S’’ For Collector-Base Bias S’’ Ic   = Ico, Vbe constant Vcc = (Ib + Ic)Rc + IbRb + Vbe Vcc –Vbe = (Ib + Ic)Rc + IbRb = Ib [(1 + )Rc +Rb] Ib = . . . Vcc – Vbe (1 + ) Rc + Rb Ic = . . . ( Vcc – Vbe) (1 + ) Rc + Rb

. . . Ic  = [(1 + )Rc +Rb](Vcc –Vbe) - (Vcc –Vbe) Rc [(1 + ) Rc + Rb]2 (Vcc –Vbe)[(1 + )Rc +Rb] - Rc [(1 + ) Rc + Rb]2 = (Vcc –Vbe)(Rc +Rb) [(1 + ) Rc + Rb]2 = = Vcc – Vbe (1 + ) Rc + Rb Rc + Rb x = Ib(Rc + Rb) (1 + ) Rc + Rb = Ic(Rc + Rb) [(1 + ) Rc + Rb] . . . S’’

If S is small, S’’ will also be small
= Ic(Rc + Rb) [(1 + ) Rc + Rb] S’’ = (Rc + Rb) (1 + ) Rc + Rb Ic  1+  = (1 + ) Rc + Rb Ic  1 1+  (Rc + Rb) (1+ ) = Ic  S 1+  If S is small, S’’ will also be small Hence if we provide stability against Ico variations, it will take care of  variation as well

For Voltage Divider Bias
Stability Factor S For Voltage Divider Bias S Ic Ico = Vbe,  constant Vb = IbRb +Vbe + IeRe = IbRb +Vbe + (Ib + Ic)Re where Rb = Rb1 ll Rb2 Differentiating, 0 = IbRb IbRe + IcRe i.e. Ib(Rb + Re) = - IcRe S = (I + ) 1 -  Ib Ic = (I + ) 1 +  Re Re + Rb Ib Ic . . . -Re Rb + Re =

In the above equation, if Rb << Re, then S becomes 1
1 +  Re Re + Rb In the above equation, if Rb << Re, then S becomes 1 Rb = Rb1 ll Rb2 Hence either Rb1 or Rb2 must be << Re Since Vb << Vcc, Rb2 is kept small wrt Rb1

If Rb-Re ratio is fixed, and if Rb >> Re, S increases with 
1 +  Re Re + Rb S = (I + ) 1 +  1 1 + Rb/Re S = (I + ) Re cannot be increased beyond a limit, as it will affect Ic and hence the Q point If Rb-Re ratio is fixed, and if Rb >> Re, S increases with  Thus stability decreases with increasing 

If Rb << Re, then S becomes independent of 
1 +  Re Re + Rb S = (I + ) 1 +  1 1 + Rb/Re S = I If Rb << Re, then S becomes independent of  Stability factor S for Voltage Divider circuit is less compared to other circuits Hence it is preferred over other circuits

For Voltage Divider Bias
Stability Factor S’ For Voltage Divider Bias Vb = IbRb +Vbe + IeRe S’ Ic Vbe = Ico,  constant = IbRb + Vbe + (Ib + Ic)Re = Ib(Rb + Re) + Vbe + IcRe = Ic /  (Rb +Re) + Vbe + IcRe Or, Vb = Ic(Rb +Re) +  Vbe +  IcRe = Ic[Rb +( + 1)Re] +  Vbe 0 = Ic[Rb +( + 1)Re] + Vbe Differentiating, Or, Vbe = - Ic [Rb +( + 1)Re] = Rb + ( + 1) Re -  S’ Ic Vbe =

Stability Factor S’’ S’’   = Vb = IbRb +Vbe + IeRe
For Voltage Divider Bias S’’ Ic   = Ico, Vbe constant Vb = IbRb +Vbe + IeRe = Ib(Rb + Re) + Vbe + IcRe = Ic /  (Rb +Re) + Vbe + IcRe Or, Vb = Ic(Rb +Re) +  Vbe +  IcRe Or, (Vb – Vbe) = Ic(Rb +Re) +  IcRe Differentiating, (Vb – Vbe) = Ic(Rb +Re) + IcRe + Ic Re (Vb – Vbe – IcRe) = Ic[Rb + Re+ Re] . . . Ic  = S’’ = Vb – Vbe - IcRe Rb + Re(1+ )

Hence Rb / Re must be small to make S’’ smaller
Ic  = S’’ = Vb – Vbe - IcRe Rb + Re(1+ ) = Vb – Vbe - IeRe Rb + Re(1+ ) As Ie = Ic = Ib Rb Rb + Re(1+ ) = Ib 1 +(Re/Rb)(1+ ) Hence Rb / Re must be small to make S’’ smaller

Bias Compensation The biasing circuits seen so far provide stability of operating point for any change in Ico, Vbe or  The collector- base bias & emitter bias circuits provide negative feedback & make the circuit stable, but the gain falls down In such cases it is necessary to use compensation techniques

Diode Compensation Technique
Vcc Here diode D has been connected as shown It is given forward bias through Vdd The diode D is identical to the BE junction of the transistor The charge carriers will increase in the BE jn. due to temperature or other variations Rb Rc 270 K 5.6 K Rd Re - D Vdd +

Rb 270 K 5.6 K Vcc Rc Rd + - Re Vdd D Since diode D has similar properties, its charge carrier also increases, for any change in the parameters Thus the increase in current in the BE junction is compensated by the current flow through the diode in the reverse direction.

Vcc Another technique Here the diode D has been connected in the bleeder path When there is increase in current in the BE junction due to parameter changes, current through D also increases by the same amount Ib1 Rb1 Rc 270 K 5.6 K D Ib2 Re Rb2

This increases Ib1, produces more drop across Rb1& reduces Vb
270 K 5.6 K Vcc Rc Re Rb2 D This increases Ib1, produces more drop across Rb1& reduces Vb As Vb decreases, Ib falls down Thus the transistor currents are arrested and not allowed to increase Thus diode D provides suitable compensation

Thermistor Compensation
270 K 5.6 K Vcc Re Rc Rb1 NTC Ib Here a Negative Temperature Coefficient Resistor has been used As temperature increases, its resistance decreases This increases Ib1 & voltage drop across Rb1 This decreases Vb and hence Ib & Ic, thus keeping the circuit stable. Ib1 Ib2

Constant Current circuit
Vcc Constant Current circuit Re provides self bias Vb is fixed depending on the ratio of Rb1 & Rb2 & the value of Vcc Ve = Vb - Vbe Vbe is fixed for a transistor Hence Ve is fixed & Ie = Ve / Re is also fixed Hence it acts as a constant current circuit Rc Rb1 5.6 K Rb2 Re

For the given Si transistor find the constant current I
Problem For the given Si transistor find the constant current I Answer I = 4.22 mA Rb1 I 5.6 K 270 K Rb2 4K7 Re 2K2 -20 V

FET Biasing

Introduction For the JFET, the relationship between input and output quantities is nonlinear due to the squared term in Shockley’s equation. Nonlinear functions results in curves as obtained for transfer characteristic of a JFET. Graphical approach will be used to examine the dc analysis for FET because it is most popularly used rather than mathematical approach The input of BJT and FET controlling variables are the current and the voltage levels respectively

Introduction -contd… JFETs differ from BJTs:
Nonlinear relationship between input (VGS) and output (ID) JFETs are voltage controlled devices, whereas BJTs are current controlled

FET Biasing Common FET Biasing Circuits • JFET – Fixed – Bias
– Self-Bias – Voltage-Divider Bias • Depletion-Type MOSFET • Enhancement-Type MOSFET – Feedback Configuration

General Relationships
For all FETs: For JFETs and Depletion-Type MOSFETs: For Enhancement-Type MOSFETs:

Fixed-Bias Configuration
The configuration includes the ac levels Vi and Vo and the coupling capacitors. The resistor is present to ensure that Vi appears at the input to the FET amplifier for the AC analysis.

Fixed-Bias Configuration-contd…
Investigating the input loop IG=0A, therefore VRG=IGRG=0V Applying KVL for the input loop, -VGG-VGS=0 VGG= -VGS It is called fixed-bias configuration due to VGG is a fixed power supply so VGS is fixed The resulting current,

Fixed-Bias Configuration(Graphical approach)
Investigating the graphical approach. Using below tables, we can draw the graph

Self Bias Configuration
The self-bias configuration eliminates the need for two dc supplies. The controlling VGS is now determined by the voltage across the resistor RS

Self Bias Configuration-contd…
For the indicated input loop: Mathematical approach:

Graphical approach Draw the device transfer characteristic
Draw the network load line First point, Second point, any point from ID = 0 to ID = IDSS. Choose the quiescent point obtained at the intersection of the straight line plot and the device characteristic curve. The quiescent value for ID and VGS can then be determined and used to find the other quantities of interest.

Graphical approach

Self Bias For output loop Apply KVL of output loop Use ID = IS

Voltage-Divider Bias The arrangement is the same as BJT but the DC analysis is different In BJT, IB provide link to input and output circuit, in FET VGS does the same

Voltage-Divider Bias The source VDD was separated into two equivalent sources to permit a further separation of the input and output regions of the network. IG = 0A ,Kirchoff’s current law requires that IR1= IR2 and the series equivalent circuit appearing to the left of the figure can be used to find the level of VG.

Voltage-Divider Bias VG can be found using the voltage divider rule :
Using Kirchoff’s Law on the input loop: Rearranging and using ID =IS: Again the Q point needs to be established by plotting a line that intersects the transfer curve.

Procedures for plotting
1. Plot the line: By plotting two points: VGS = VG, ID =0 and VGS = 0, ID = VG/RS 2. Plot the transfer curve by plotting IDSS, VP and calculated values of ID. 3. Where the line intersects the transfer curve is the Q point for the circuit.

Voltage Divider Bias Once the quiescent values of IDQ and VGSQ are determined, the remaining network analysis can be found. Output loop:

Effect of increasing values of RS

Depletion-Type MOSFETs
Depletion-type MOSFET bias circuits are similar to JFETs. The only difference is that the depletion-Type MOSFETs can operate with positive values of VGS and with ID values that exceed IDSS.

Analyzing the MOSFET circuit for DC analysis
How to analyze dc analysis for the shown network? It is a Type network Find VG or VGS Draw the linear characteristics Draw the transfer characteristics Obtain VGSQ and IDQ from the graph intersection

Depletion type MOSFET 1. Plot line for VGS = VG, ID = 0 and ID = VG/RS, VGS = 0 2. Plot the transfer curve by plotting IDSS, VP and calculated values of ID. 3. Where the line intersects the transfer curve is the Q-point. Use the ID at the Q-point to solve for the other variables in the voltage-divider bias circuit. These are the same calculations as used by a JFET circuit.

Q-Point- Enhancement MOSFET
1. Plot line for VGS = VG, ID = 0 and ID = VG/RS, VGS = 0 2. Plot the transfer curve by plotting IDSS, VP and calculated values of ID. 3. Where the line intersects the transfer curve is the Q-point. Use the ID at the Q-point to solve for the other variables in the voltage-divider bias circuit. These are the same calculations as used by a JFET circuit.

Enhancement-Type MOSFET
The transfer characteristic for the enhancement-type MOSFET is very different from that of a simple JFET or the depletion-type MOSFET.

Enhancement MOSFET Transfer characteristic for E-MOSFET

Feedback Biasing Arrangement
IG =0A, therefore VRG = 0V Therefore: VDS = VGS Which makes

Feedback Biasing Q-Point
1. Plot the line using VGS = VDD, ID = 0 and ID = VDD / RD and VGS = 0 2. Plot the transfer curve using VGSTh , ID = 0 and VGS(on), ID(on); all given in the specification sheet. 3. Where the line and the transfer curve intersect is the Q-Point. 4. Using the value of ID at the Q-point, solve for the other variables in the bias circuit.

DC analysis step for Feedback Biasing Enhancement type MOSFET
Find k using the datasheet or specification given; ex: VGS(ON),VGS(TH) Plot transfer characteristics using the formula ID=k(VGS – VT)2. Three point already defined that is ID(ON), VGS(ON) and VGS(TH) Plot a point that is slightly greater than VGS Plot the linear characteristics (network bias line) The intersection defines the Q-point

Voltage-Divider Biasing
Again plot the line and the transfer curve to find the Q-point. Using the following equations: Input loop : Output loop :

Voltage-Divider Bias Q-Point
Plot the line using VGS = VG = (R2VDD)/(R1 + R2), ID = 0 and ID = VG/RS and VGS = 0 Find k 3. Plot the transfer curve using VGSTh, ID = 0 and VGS(on), ID(on); all given in the specification sheet. 4. Where the line and the transfer curve intersect is the Q-Point. 5. Using the value of ID at the Q-point, solve for the other variables in the bias circuit.

Troubleshooting N-channel VGSQ will be 0V or negative if properly checked Level of VDS is ranging from 25%~75% of VDD. If 0V indicated, there’s problem Check with the calculation between each terminal and ground. There must be a reading, RG will be excluded

P-Channel FETs For p-channel FETs the same calculations and graphs are used, except that the voltage polarities and current directions are the opposite. The graphs will be mirrors of the n-channel graphs.

Practical Applications
• Voltage-Controlled Resistor • JFET Voltmeter • Timer Network • Fiber Optic Circuitry • MOSFET Relay Driver

Power Supplies Biasing BJT and MOSFET

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Power Supplies Biasing BJT and MOSFET

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