The Common Emitter Amplifier : Mycircuitstudy

The Common Emitter Amplifier

The common emitter configuration is the most versatile of the three. It has low input

impedance, moderate output impedance, voltage gain and current gain. The input and

output are often capacitively coupled. Before performing an AC analysis we will discuss DC biasing.

1. DC Biasing

DC biasing is setting up a circuit to operate a transistor at a desired operating point on its characteristic curve. Three bias networks for the common emitter amplifier are shown in  the only path for DC bias current into the base is through RB. VCC is a power supply voltage which is generally greater than 10 V such that VPN can be ignored.

The DC voltage at the collector should be large enough to provide at least a 2 V drop between collector and emitter and clearly must be less than VCC. In the absence of other circuit requirements, a convenient algebraic choice for VC is VCC/2. DC circuit analysis results in the following relative sizes of the two resistors: RB = 2hFERC.

Although the circuit works reasonably well, the fact that hFE is quite variable among

samples leads to a bad design. A well-designed circuit should have an operating point that is less dependent on this parameter. shows a network with the base-biasing resistor connected to the collector instead of VCC. RF acts as a negative feedback resistor since it feeds the collector current back into the base. Analysis gives

RF = hFERC.

Therefore a change in hFE has only half the effect of the previous design.

A more common bias stabilization technique employs a series resistor between the emitter and ground. This circuit has about the same sensitivity to changes in hFE as the previous

circuit.

Bias circuits for the common emitter amplifier.

A further improvement can be made by introducing a second base-bias resistor as shown in figure . The bias voltage is determined almost entirely by the two bias resistors.

These biasing methods can also be used for the common collector and common base configurations.

2. Approximate AC Model

The circuit  is the basic common emitter amplifier using the simplest biasing method. Because it is constant, the power supply voltage VCC is an AC ground indistinguishable from the normal ground of the circuit. We can therefore relocate the upper end of RB and RC to the common ground line .

The transistor symbol is ideal and hie is shown explicitly as the input impedance and hence iS = iB.

3.The Basic CE Amplifier

We can now use the AC equivalent circuit to calculate the AC voltage gain between the base and collector. The base voltage is developed across the input resistor hie and vB = hieiB.

The collector voltage can be similarity expressed as the voltage drop across the resistor RC: 0 − vC = RChfeiB. Eliminating iB, we can write the amplifier voltage transfer function

between the base and collector .

 a) Basic CE amplifier and b) AC equivalent circuit drawn using an ideal transistor

symbol with hie shown explicitly.

The minus sign indicates that the voltage signal at the collector is 180o out of phase with

the signal at the base. The input impedance to this amplifier circuit is just the parallel combination of RB and hie, and since hie is usually much smaller than RB, the input impedance generally reduces to just the input impedance of the transistor itself, namely, hie.

The circuit output impedance is the collector resistance RC.

The high-frequency operation of the common emitter amplifier is limited by the parasitic

capacitance between the collector and base. This capacitance provides a path by which

the large and inverted signal at the collector drives a feedback current into the base. The base-to-collector voltage gain of this amplifier looks like a low-pass filter.