Common Emitter Characteristics

Common Emitter Characteristics

Transistor Amplifier Basics

It is critical to understand the notation used for voltages and currents in the following discussion of transistor amplifiers. This is therefore dealt with explicitly ‘up front’. As with dynamic resistance in diodes we will be dealing with a.c. signals superimposed on d.c. bias levels.

We will use a capital (upper case) letter for a d.c. quantity (e.g. I, V).. We will use a lower case letter for a time varying (a.c.) quantity (e.g. i, v)

These primary quantities will also need a subscript identifier (e.g. is it the base current or the collector current?).. For d.c. levels this subscript will be in upper case. We will use a lower case subscript for the a.c. signal bit (e.g. i_b). And anupper case subscript for the total time varying signal (i.e. the a.c. signal bit plusthe d.c. bias) (e.g. i_B).This will be less common.

Transistor Amplifier BasicsTransistor Amplifier Basics

It is convention to refer all transistor voltages to the ‘common’ terminal. Thus in the CE configuration we would write V_CE for a d.c. collector emitter voltage and V_BEfor a d.c. base emitter voltage.

Common Emitter Characteristics

For the present we consider DC behaviour and assume that we are working in the normal linear amplifier regime with the BE junction forward biased and the CB junction reverse biased

Treating the transistor as a current node:

Also:

Hence:

which after some rearrangement gives        

Define a common emitter current-transfer ratio b 

Such that:

Since reverse saturation current is negligible the second term on the right hand side of this equation can usually be neglected (even though (1- α)  is small) thus

We note, in passing that, if β can be regarded as a constant for a given transistor then 

For a practical (non-ideal) transistor this is only true at a particular bias (operating) point.

A small change in α causes a much bigger change in ß which means that ß can vary significantly, even from transistor to transistor of the same type. We must try and allow for these variations in circuit design.

For example;

α  = 0.98, β = 49

α  = 0.99, β = 99

α = 0.995, β =199

b is also known as h_FE and may appear on data sheets and in some textbooks as such. For a given transistor type data sheets may specify a range of b values.

The behaviour of the transistor can be represented by current-voltage (I-V) curves (called the characteristic curves of the device). As noted previously in the common emitter (CE) configuration the input is between the base and the emitter and the output is between the collector and the emitter.

We can therefore draw an input characteristic (plotting base current I_B against base-emitter voltage V_BE) and an output characteristic (plotting collector current I_c against collector-emitter voltage V_CE)

We will be using these characteristic curves extensively to understand: How the transistor operates as a linear amplifier for a.c. signals.

The need to superimpose the a.c. signals on d.c. bias levels. The relationship between the transistor and the circuit in which it is placed.

Once these basics are understood we will understand. Why we can replace the transistor by a small signal (a.c.) equivalent circuit. How to derive a simple a.c. equivalent circuit from the characteristic curves. Some of the limitations of our simple equivalent circuit.

IDEAL CE  INPUT Characteristics

The plot is essentially that of a forward biased diode. We can thus assume V_BE » 0.6 V when designing our d.c. bias circuits. We can also assume everything we know about incremental diode resistance when deriving our a.c. equivalent circuit. In the ‘non-ideal’ case I_B will vary slightly with V_CE. This need not concern us.

IDEAL CE  OUTPUT (Collector) Characteristics

IDEAL CE  OUTPUT

Avoid this cut-off region where we try to reverse bias both junctions (I_C approximately 0)

The plots are all parallel to the V_CE axis (i.e. I_C does not depend on V_CE). The curves strictly obey I_C = βI_B

In particular I_C = 0 when I_B = 0. We shall work with the ideal characteristic and later on base our a.c. equivalent circuit model upon it. 

ACTUAL CE  OUPUT Characteristics

Salient features are:

The finite slope of the plots (I_C depends on V_CE)_. A limit on the power that can be dissipated. The curves are not equally spaced (i.e β varies with base current, I_B).

You will get to measure these curves in the lab. There is also a PSPICE sheet “DC sweep analysis and transistor characteristics” to help aid you understanding.