Download presentation

Published byPeter Rich Modified over 6 years ago

1
**Recall Lecture 13 Biasing of BJT Applications of BJT**

Three types of biasing Fixed Bias Biasing Circuit Biasing using Collector to Base Feedback Resistor Voltage Divider Biasing Circuit Applications of BJT As digital logic gates NOT NOR

2
**Chapter 5 basic bjt amplifiers (AC ANALYSIS)**

3
**The Bipolar Linear Amplifier**

Bipolar transistors have been traditionally used in linear amplifier circuits because of their relatively high gain. To use the circuit as an amplifier, the transistor needs to be biased with a dc voltage at a quiescent point (Q-point) such that the transistor is biased in the forward-active region. If a time-varying signal is superimposed on the dc input voltage, the output voltage will change along the transfer curve producing a time-varying output voltage. If the time-varying output voltage is directly proportional to and larger than the time-varying input voltage, then the circuit is a linear amplifier.

4
**The linear amplifier applies superposition principle**

Response – sum of responses of the circuit for each input signals alone So, for linear amplifier, DC analysis is performed with AC source turns off or set to zero AC analysis is performed with DC source set to zero

5
EXAMPLE iC , iB and iE, vCE and vBE Sum of both ac and dc components

6
**Graphical Analysis and ac Equivalent Circuit**

From the concept of small signal, all the time-varying signals are superimposed on dc values. Then: and

7
**PERFORMING DC and AC analysis**

DC ANALYSIS AC ANALYSIS Turn off DC SUPPLY = short circuit Turn off AC SUPPLY = short circuit

8
DO YOU STILL REMEMBER?

9
**Let’s assume that Model 2 is used**

IDQ VDQ = V rd id DC equivalent AC equivalent

10
**AC equivalent circuit –**

Small-Signal Hybrid-π Equivalent ib OR

11
**THE SMALL SIGNAL PARAMETERS**

The resistance rπ is called diffusion resistance or B-E input resistance. It is connected between Base and Emitter terminals The term gm is called a transconductance ro = VA / ICQ rO = small signal transistor output resistance VA is normally equals to , hence, if that is the case, rO = open circuit

12
Hence from the equation of the AC parameters, we HAVE to perform DC analysis first in order to calculate them.

13
EXAMPLE The transistor parameter are = 125 and VA=200V. A value of gm = 200 mA/V is desired. Determine the collector current, ICQ and then find r and ro ANSWERS: ICQ = 5.2 mA, r= k and ro = 38.5 k

14
**Voltage Gain, AV = vo / vs Current Gain, Ai = iout / is**

CALCULATION OF GAIN Voltage Gain, AV = vo / vs Current Gain, Ai = iout / is

15
**Small-Signal Voltage Gain: Av = Vo / Vs**

ib

16
**Common-Emitter Amplifier**

17
**Remember that for Common Emitter Amplifier,**

the output is measured at the collector terminal. the gain is a negative value Three types of common emitter Emitter grounded With RE With bypass capacitor CE

18
STEPS OUTPUT SIDE Get the equivalent resistance at the output side, ROUT Get the vo equation where vo = - ib ROUT INPUT SIDE Calculate Rib using KVL where Rib = vb / ib Calculate Ri Get vb in terms of vs – eg: using voltage divider. Go back to vo equation and replace where necessary

19
**Emitter Grounded β = 100 VBE = 0.7V VA = 100 V**

VCC = 12 V RC = 6 k 93.7 k 6.3 k 0.5 k β = 100 VBE = 0.7V VA = 100 V Voltage Divider biasing: Change to Thevenin Equivalent RTH = 5.9 k VTH = V

20
**Perform DC analysis to obtain the value of IC**

BE loop: 5.9IB – = 0 IB = IC = βIB = mA Calculate the small-signal parameters r = 2.74 k , ro = k and gm = 36.5 mA/V

21
**Emitter Grounded β = 100 VBE = 0.7V VA = 100 V VCC = 12 V RC = 6 k**

22
**Follow the steps 1. Rout = ro || RC = 5.677 k**

vb Follow the steps 1. Rout = ro || RC = k 2. Equation of vo : vo = - ( ro || RC ) ib= ib 3. Calculate Rib using KVL: ib r - vb = 0 hence Rib = r 4. Calculate Ri RTH||r = 1.87 k 5. vb in terms of vs use voltage divider: vb = [ Ri / ( Ri + Rs )] * vs = vs

23
**so: vb = 0.789 vs replace in equation from step 1 **

6. Go back to equation of vo vo = ib but ib = vb / Rib (from step 3) vo = [0.789 vs] / 2.74 vo = vs AV = vo / vs = bring VS over

24
**TYPE 2: Emitter terminal connected with RE – normally ro = in this type**

β = 120 VBE = 0.7V VA = VCC = 5 V RC = 5.6 k 250 k 75 k 0.5 k RE = 0.6 k

25
0.5 k 57.7 k RC = 6 k 7.46 k RE = 0.6 k vb

26
**2. Equation of vo : vo = - RC ib= - 720 ib**

vb 1. Rout = RC = 6 k 2. Equation of vo : vo = - RC ib= ib 3. Calculate Rib using KVL: ib r + ie RE - vb = 0 but ie = (1+ ) ib = 121 ib so: ib [ 121(0.6) ] = vb Rib = k 4. Calculate Ri RTH||Rib = k 5. vb in terms of vs use voltage divider: vb = [ Ri / ( Ri + Rs )] * vs = vs

27
**6. Go back to equation of vo vo = - 720 ib = - 720 [ vb / Rib ] **

so: vb = vs 6. Go back to equation of vo vo = ib = [ vb / Rib ] vo = [ vs / ] vo = vs AV = vo / vs = bring VS over

28
**TYPE 3: With Emitter Bypass Capacitor, CE**

Circuit with Emitter Bypass Capacitor There may be times when the emitter resistor must be large for the purpose of DC design, but degrades the small-signal gain too severely. An emitter bypass capacitor can be used to effectively create a short circuit path during ac analysis hence avoiding the effect RE

29
vb CE becomes a short circuit path – bypass RE; hence similar to Type 1

30
**β = 125 VBE = 0.7V VA = 200 V IC = 0.84 mA Bypass capacitor VCC = 5 V**

RC = 2.3 k 20 k 0 k RE = 5k Bypass capacitor

31
**Short-circuited (bypass) by the capacitor CE**

β = 125 VBE = 0.7V VA = 200 V IC = 0.84 mA 0 k 10 k RC = 2.3 k 238 k 3.87 k vb Short-circuited (bypass) by the capacitor CE

32
**Follow the steps 1. Rout = ro || RC = 2.278 k**

vb Follow the steps 1. Rout = ro || RC = k 2. Equation of vo : vo = - ( ro || RC ) ib= ib 3. Calculate Rib using KVL: ib r - vb = 0 hence Rib = r = 3.87 k 4. Calculate Ri RTH||r = 2.79 k 5. vb in terms of vs use voltage divider: vb = [ Ri / ( Ri + Rs )] * vs = vs since RS = 0 k

33
**6. Go back to equation of vo vo = - 284.75 ib = - 284.75 [ vb / Rib ] **

so: v b = vs 6. Go back to equation of vo vo = ib = [ vb / Rib ] vo = (vs) / 3.87 vo = vs AV = vo / vs = bring VS over

Similar presentations

© 2021 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google