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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.

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ce amplifiers2
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 amplifiers3
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 bias5
CE AMPLIFIER, Simple bias

+VCC

IC

IB

RB

RC

VCE

VBE

GND

ce amplifier simple bias6
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 bias7
CE AMPLIFIER, Simple bias

+5

IC

RB

2.5 x 103

GND

ce amplifier simple bias8
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 bias9
CE AMPLIFIER, Simple bias

+5

IC

RB

2.5 x 103

GND

ce amplifier simple bias10

+VCC

IC

RC

VCE

GND

CE AMPLIFIER, Simple bias

From Kirchhoff, for the output,

ce amplifier simple bias11
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 VCEimposed by the circuit or load.
ce amplifier simple bias12
CE AMPLIFIER, Simple bias
  • Another variation of IC with VCE is determined by the output characteristic.
ce amplifier simple bias13
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 bias14
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.

CE AMPLIFIER, Simple bias
ce amplifier simple bias16
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 bias17
CE AMPLIFIER, Simple bias

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

ce amplifier simple bias18

+VCC

IB

RB

VBE

GND

CE AMPLIFIER, Simple bias
  • From Kirchhoff, for the input,
ce amplifier simple bias19
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 bias20
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

VCC

IC

RL

RB

VCE

RS

VS

GND

CE AMPLIFIER

RC

Signal input

Signal output

ce amplifier22
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 amplifier23
CE Amplifier
  • To find the voltage and current gains using the load line method we must use the input and output characteristics.
ce amplifier24
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 amplifier25
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 amplifier26
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 amplifier27
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 amplifier28
CE AMPLIFIER

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

ce amplifier29
CE Amplifier
  • The CE small signal (a.c.) voltage gain is
ce amplifier30
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 amplifier31
CE AMPLIFIER

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

ideal ce amplifier summary
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 summary33
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

vout

vin

RL

GND

Additional a.c. Load
  • Suppose an a.c. coupled load RL = 2.5 kΩ is added

VCC

RC

C

additional a c load35
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 load36
Additional a.c. load

a.c. short via d.c. supply

RC

iC

RL

GND

additional a c load38
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 load39
Additional a.c. Load

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

additional a c load40
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
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

VCC

RC

R1

vout

RS

R2

RE

VS

GND

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

stabilised bias circuit43
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 circuit44
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 circuit45
Stabilised Bias Circuit

+VCC

IRC

RC

RB

IC

D.C collector voltage VC

IB

VBE =0.6 V

GND

stabilised bias circuit46
Stabilised Bias Circuit

+VCC

IRC

RC

RB

IC

D.C collector voltage VC

IB

VBE =0.6 V

GND

stabilised bias circuit47
For example, increasing , increases IC which lowers the collector voltage VC and hence and IB andICStabilised Bias Circuit

+VCC

RC

RB

IC

D.C collector voltage VC

IB

VBE =0.6 V

GND