1. Transistors Fundamentals Common-Emitter Amplifier What transistors do
How to analyse transistor circuits
Small and large signals
Common-Emitter Amplifier
Review of analysis and design

2. The Bipolar Junction Transistor
BJT is a current amplifier
The collector current is controlled by a much smaller base current
The sum of the collector and base currents flow into or out of the emitter
Base-emitter junction looks a lot like a PN junction diode

3. Operating Regions - Cut Off
If the base current is zero, the collector current is also zero
It doesn’t matter how big the collector-emitter voltage, VCE, is
i.e. collector-emitter junction looks like an open circuit
In this state, the transistor is in the cut-off region

4. Operating Regions - Active
Base current flows and controls the larger collector current
Collector current is proportional to the base current
Transistor is in the active region
Operation can be summarised by two equations:
VBE IC IB

5. Operating Regions - Saturation
Collector current rises in proportion to the base current
As collector current rises, resistor voltage rises and collector-emitter voltage falls
When VCE » 0, it can’t go any lower and the collector current cannot get any higher
The transistor is saturated
Collector-emitter junction looks like a short circuit

6. Amplification
BJT amplifiers work by controlling the collector current by the base-emitter voltage
This is only possible in the active region
Cut-off and saturation regions correspond to the transistor turning fully ‘off’ or ‘on’ like a switch
In the active region, the transistor is only partly ‘on’ and the current can be controlled

7. Small Signals
We want circuits with a linear response but real transistors aren’t linear
Current
If the range of voltages/currents is kept small, response is approximately linear DI DV
= v
= i
Small variations (i.e. signals!) are denoted by lower case V I
Average (or quiescent) levels are denoted by capital letters
Voltage

8. Small Signal Collector Current

9. Mutual Conductance IC and VBE are exponentially related
iC and vBE, on the other hand, are approximately linearly related
The constant of proportionality, gm, is known as the mutual conductance
It isn’t a real conductance, but it is the ratio between a current and a voltage

10. Estimating gm
The small signal behaviour is estimated by a tangent to the exponential IC-VBE curve
gm is, therefore, simply the gradient of the curve

11. Amplification
Assume that the transistor is biased in the active region somehow…
Collector voltage, VC, is related to IC by Ohm’s law RC IC
Small signal ratio between collector voltage and collector current is: VC
VBE
So:

12. Simple Common-Emitter Amplifier
IB provides a d.c. base current to bias the transistor in the active region
CIN couples the input voltage, removing the d.c. base bias voltage
CIN is a short circuit to a.c. signals…
…but an open circuit to the d.c. bias current
vBE is, therefore, equal to vIN

13. Analysis

14. Biasing
Gain is proportional to gm which is, in turn, proportional to IC
In this circuit,
Unfortunately, b has a very wide tolerance
The gain is, therefore, not predictable

15. Reliable Biasing Collector current is set accurately regardless of b
CE ensures that the whole of the a.c. input voltage is still dropped across VBE
RB provides the d.c. base bias current
Usually, the current source is approximated by a resistor

16. Practical Amplifier To analyse the circuit:
Determine quiescent conditions
Calculate mutual conductance
Calculate small signal performance
Voltage Gain
Input Impedance
Output Impedance
Cut-off frequency

17. The Story so Far
Small signal analysis is used to simplify calculations by ‘linearising’ the non-linear response of the transistor
Using mutual conductance, gain calculations are now only a couple of lines of equations
Careful choice of the biasing network leads to reliable performance
Next time – practical amplifier calculations, input & output impedances and capacitor calculations