CE AMPLIFIERS

CE AMPLIFIERS The first step is to set up an operating or ‘Q’ point using a suitable bias circuit. We will, by way of introduction, use a so called load line technique to see the interplay between the circuit and device constraints on voltage and current. This will provide a graphical analysis of amplifier behaviour.

CE AMPLIFIERS The following (simple) bias circuit uses a single resistor RB to fix the base current. It is not very good since the emitter/collector currents and hence the operating point (IC, VCE) vary with β. This will be improved with stabilised bias circuits in due course.

CE AMPLIFIER, Simple bias +VCC IC IB RB RC GND

CE AMPLIFIER, Simple bias +VCC IC IB RB RC VCE VBE GND

CE AMPLIFIER, Simple bias To enable us to look at a particular numerical example we choose the supply voltage VCC = 5V and RC = 2.5 kΩ

CE AMPLIFIER, Simple bias +5 IC RB 2.5 x 103 GND

CE AMPLIFIER, Simple bias In later discussions an a.c. signal (and an additional load resistor) will be coupled to the d.c. circuit using coupling capacitors. The capacitor values are chosen so that their impedance (1/ C) is negligibly small (zero) at the a.c.(signal) frequency (or over the operating frequency range). A capacitor acts as a short circuit for d.c. and the d.c. bias circuit can be designed independently of the a.c. source and any ‘a.c. load’.

CE AMPLIFIER, Simple bias +5 IC RB 2.5 x 103 GND

CE AMPLIFIER, Simple bias From Kirchhoff, for the output, RC GND +VCC IC VCE

CE AMPLIFIER, Simple bias Numerically, 5 - 2.5 x 103 IC-VCE =0 Or, rearranging, IC = (5 – VCE )/ (2.5 x 103) A plot of IC against VCE is a straight line with slope (– 1/ 2.5 x 103) It is called a load line and represents the variation of IC with VCE imposed by the circuit or load.

CE AMPLIFIER, Simple bias Another variation of IC with VCE is determined by the output characteristic.

CE AMPLIFIER, Simple bias Another variation of IC with VCE is determined by the output characteristic. The two relationships can be solved graphically for IC and VCE.

CE AMPLIFIER, Simple bias Thus we calculate three points on the load line IC = (5 – VCE )/ (2.5 x 103) as IC =0, VCE =5V IC = 1mA, VCE =2.5V VCE =0V, IC =5/2500 A = 2mA. To enable us to plot it on the output characteristic. Two points define a straight line. It is a good idea to use 3 to plot the line in case of numerical error.

CE AMPLIFIER, Simple bias

CE AMPLIFIER, Simple bias The region along the load line includes all points between saturation and cut-off. The base current IB should be chosen to maximise the output voltage swing in the linear region. Bearing in mind that VCE (Sat)  0.2 V and VCE Max = 5V choose the operating (Q) point at IB = 10 μA.

CE AMPLIFIER, Simple bias ‘Operating’ or Q point set by d.c. bias.

CE AMPLIFIER, Simple bias From Kirchhoff, for the input, RB GND +VCC IB VBE

CE AMPLIFIER, Simple bias Remembering that VBE ~ 0.6 V (the base or input characteristic is that of a forward biased diode) we can find RB ~ 440 kΩ.

CE AMPLIFIER, Simple bias A a.c. signal is superimposed on top of the d.c. bias level. We are interested in the voltage and current gains for this a.c. component.

CE AMPLIFIER VS RS VCC GND VCE RL RB IC RC Signal input Signal output

CE AMPLIFIER The Q (d.c. bias) value of VCE is about 2.5 V The maximum positive signal swing allowed is, therefore (5-2.5) V = 2.5 V (The total The maximum negative voltage swing allowed is (2.5 –0.2) V =2.3 V The maximum symmetric symmetric signal swing about the Q point is determined by the smaller of these, i.e. it is 2.3 V.

CE Amplifier To find the voltage and current gains using the load line method we must use the input and output characteristics.

CE Amplifier Diode dynamic resistance for signals = 1/slope at Q point! Defines transistor input impedance for signals Remember we selected IB = 10 μA

CE Amplifier From the input curve we estimate that as IB changes by 5μA about the bias level of 10μA then the corresponding change in VBE is about 0.025 V. When iB =5μA, vBE = 0.5875V; when iB =15μA, vBE = 0.6125.

CE Amplifier From the output characteristic curve we move up and down the load line to estimate that as IB changes by 5μA the corresponding change in VCE is about –2.5 V. (Note the negative sign!) When iB =5μA, vCE = 3.75V; when iB =15μA, vCE = 1.25V

CE Amplifier From the input curve we estimate that as IB changes by 5μA about the bias level of 10μA then the corresponding change in VBE is about 0.025 V. When iB =5μA, vBE = 0.5875V; when iB =15μA, vBE = 0.6125.

CE AMPLIFIER ‘Operating’ or Q point set by d.c. bias.

CE Amplifier The CE small signal (a.c.) voltage gain is

CE Amplifier From the output characteristic curve we also see that as we move up and down the load line a change in IB of 5μA produces a corresponding change in IC of 5mA. The a.c. signal current gain is 100. This is consistent with the ideal characteristic uniform line spacing, i.e. β = 100 = constant.

CE AMPLIFIER ‘Operating’ or Q point set by d.c. bias.

Ideal CE Amplifier Summary The CE voltage and current gains are high The voltage gain is negative, i.e. the output signal is inverted. The d.c. bias current sets the signal input impedance of the transistor through the dynamic resistance. IC = β IB ; iC = β iB.

Ideal CE Amplifier Summary Two of these statements: The d.c. bias current sets the signal input impedance of the transistor through the dynamic resistance. IC = β IB ; iC = β iB. will be used to derive our simplified small signal equivalent circuit of the BJT. (It is simplified because it is based on ideal BJTs)

Additional a.c. Load Suppose an a.c. coupled load RL = 2.5 kΩ is added vin GND vout RL VCC RC C

Additional a.c. Load The ‘battery’ supplying the d.c. supply VCC has negligible impedance compared to the other resistors, in particular RC. It therefore presents an effective ‘short-circuit’ for a.c. signals. The effective a.c. load is the parallel combination of RC and RL . (From the collector C we can go through RC or RL to ground)

Additional a.c. load a.c. short via d.c. supply RC iC RL GND

Additional a.c. load iC RL RC vce GND

Additional a.c. Load We now need to construct an a.c. load line on the output characteristic. This goes through the operating point Q and has slope This is hard to draw!

Additional a.c. Load a.c. load line, drawn with required slope through Q point.

Additional a.c. Load The available voltage swing and the voltage gain are calculated using the a.c. loadline. Symmetric swing reduced to about 1.25 V Voltage gain reduced to about –50.

Stabilised Bias Circuits These seek to fix the emitter current independently of BJT parameter variations, principally in β. This is best achieved by introducing an emitter resistance and setting the base voltage via a resistor network (R1, R2) which acts as a potential divider (provided IB can be assumed small)

Stabilised Bias Circuit Bias bit of the circuit, a.c. source and load capacitor coupled. RE is capacitor by-passed (shorted) for a.c. signals VS RS VCC GND vout RC R1 R2 RE

Stabilised Bias Circuit See handout for a detailed analysis of this bias circuit We will also look at a worked example of a transistor amplifier based on such a stabilised bias circuit once we have established an a.c. equivalent circuit for the transistor.

Stabilised Bias Circuit Finally we give another circuit which provides bias stability using negative feedback from the collector voltage. +VCC RC RB IC D.C collector voltage VC IB VBE =0.6 V GND

Stabilised Bias Circuit +VCC IRC RC RB IC D.C collector voltage VC IB VBE =0.6 V GND

Stabilised Bias Circuit +VCC IRC RC RB IC D.C collector voltage VC IB VBE =0.6 V GND

Stabilised Bias Circuit For example, increasing , increases IC which lowers the collector voltage VC and hence and IB and IC +VCC RC RB IC D.C collector voltage VC IB VBE =0.6 V GND