As shown before, the DC operating point of a transistor amplification circuit needs to be set up properly (in the middle of the linear region) to avoid signal distortion. We now consider how the operating point is determined by the biasing circuit, in terms of , , and .
By properly setting the voltage (not too low) and (not too large), the voltage can be approximated as a constant value of , as shown in the input characteristic plot:
The DC operating point in terms of and can be found in the following steps:
If the transistor is in linear region, then
In the fixed biasing transistor circuit shown above, , , , find so that the DC operating point is in the middle of the linear region of the output characteristic plot, i.e., . We assume (may not be valid if is too large) and get
In the same circuit above, , , . Find the operating point for .
The load line is determined by these two points:
To minimize distortion, the DC operating point needs to be in the middle of the load line at . But in this case, we see that
The DC operating point of this fixed biasing circuit is not completely determined by the parameters of the circuit such as the resistors, as it is also directly affected by factors such as value and temperature. This situation can be improved by introducing negative feedback into the circuit.
To correct the problem above, the self-biasing circuit shown below can be used to decrease the effect of changing by negative feed back due to the introduction of .
Qualitatively, an increased (caused by reasons such as increased due to temperature change) will cause the following to happen:
Quantitatively, we can further carry out analysis of the circuit:
Applying KVL to the base loop we get
For this approximation above to be valid, we desire to have smaller so that is less affected by , and large for stronger negative feedback. However, as the voltage gain of the circuit will be reduced due to the negative feedback, cannot be too large.
In the circuit of self-biasing, , , , , , Assume . The load line is determined by this equation:
To minimize distortion, the desired operating point should be in the middle of the load line at and .
We see that in all three cases, , , i.e., the DC operating point is always close to the middle of the load line.
In a self-biasing transistor circuit, , , , , find so that the DC operating point is in the middle of the linear region of the output characteristic plot.
We first convert the base circuit into its Thevenin's equivalent voltage source composed of
The circuit below shows yet another way to introduce feedback to stablize the DC operating point.
We want the DC operating point to be at and , and get
As discussed above, to avoid distortion of the AC signal, we desire that the DC operating point is in the middle of the linear region of the output characterisc. For example, in the fixed biasing circuit, we want and . The corresponding power consumption is therefor , even when the AC signal is zero.