Electronics Engineering And Circuit Design

CE input, output characteristics, input resistance, output resistance, current gain, voltage gain and power gain

Look at the figure 1(a), input is applied to the base terminal and output is obtained from collector. Whereas emitter is a common terminal to both the input and output signal. This is common emitter configuration. The common emitter configuration has medium input impedance and medium output impedance. Both the current and voltage gains are high.

To understand the complete behaviour of a transistor, we need to analyse the input and output characteristics. For this analysis, the transistor must be in active region.

Biasing:

How to design or bias a CE transistor configuration?

Here are a few steps to follow.

Indicate all the directions of current (consider non transistor)
Introduce supply voltage. The polarities of the supply should support the directions of IB and IC
Here we consider electron current (the electron current flow from negative to positive terminal)
Repeat the same procedure for pnp transistor. For pnp transistor all the polarities are reversed

So, look at the complete circuit shown in figure 1(a) and
1(b)

Common Emitter Configuration Input and output characteristics curves

Figure: Common Emitter Configuration - Input and output characteristics and biasing

Supply voltages are such that they support the direction of transistor currents. (Means the direction of current from supply is in the same direction as that of transistor currents). Positive terminal of the supply is connected to the base, makes the base-emitter junction forward biased. While the positive terminal of the supply is connected to collector, makes the base-collector junction reverse biased.

In common emitter configuration:

Input voltage is base-emitter voltage
input current is IB
Output voltage is collector-emitter voltage
output current is IC

Input Characteristics:

The graph plotted between base current IB and the base emitter voltage VBE at constant collector emitter voltage VCE is called input characteristics. Base current IB is taken along y- axis (dependent variable) and base emitter voltage VBE along x-axis.

From figure 1(c) we can conclude that:

The input characteristics curve of CE configuration resembles the diode characteristics curve
After threshold voltage, the small change in VBE, IB increases rapidly
Small dynamic input resistance. It is the ratio of change in VBE to the resulting change in IB , for a given value of collector emitter voltage.

ri = ∆VBE / ∆IB | VCE = constant

Output Characteristics:

The graph plotted between collector current IC and collector emitter voltage VCE at constant values of base current IB is called output characteristics. Collector current IC is taken along y-axis and collector emitter voltage VCE is taken along x-axis.

From figure 1(d) we can conclude that:

The value of βdc is simply calculated by the ratio of IC to IB. For βac we take the ratio of small change in IC (∆IC) to the small change in IB (∆IB). It means dynamic output resistance of CE configuration is high

βac = ∆IC / ∆IB | ∆VCE = 0

From the graph you can observe, the change in VCE will cause little change in IC for constant IB. It means dynamic output resistance of CE configuration is high

rO = ∆VCE / ∆IC | IB = constant

The output characteristics curve has all three regions, active, saturation and cut-off

Current Amplification:

As you know the current amplification factor is defined by the ratio of output current to the input current. Here input current is IB and output current is IC. Hence the current amplification factor is

β = IC / IB

Since IB is very small, so the value of β is quite high.

Input Resistance:

Input resistance of CE is the ratio of base to emitter voltage VBE to the base current IB.

Ri = VBE / IB

IB is negligibly small and VBE is also a small value (VBE =0.7V). So, the input resistance is moderately high.

Output Resistance:

In this configuration, output resistance is the ratio of base to collector voltage VCE to the output current IC.

RO = VCE / IC

Voltage Gain:

Voltage gain is the ratio of output voltage to the input voltage. Input voltage is the small forward base emitter voltage (VBE = 0.7V). Output voltage is the voltage across collector terminal and it is quite high. So, the voltage gain is high for CE configuration.

AV = vce / vbe = ic*Rc

AV = β*ib*RL / ib Ri

Av = β*RL / Ri