The CE amplifier is often constructed with an emitter resistor as shown in figure 5.9. This resistor provides a form of negative feedback that can be used to stabilize both the DC operating point and the AC gain. It can be shown that the voltage transfer function across the transistor is
Figure 5.9: CE amplifier with emitter resistor.
If , the gain becomes independent of the hybrid parameters:
Because it is unaffected by variations in the hybrid parameters, this result is valid even for a large-amplitude signal.
The input resistance can be shown to be . Since is small, it can usually be neglected whenever the emitter impedance is more than a few hundred ohms.
For small input signals it is often desirable to retain the large voltage gain of the basic CE amplifier even though an emitter resistor is used for DC stability. This can be done if a large capacitor is used to bypass the AC signal around the emitter resistor. shorts out the emitter bias resistor for AC signals. The magnitude of the resulting transfer function is similar to a high-pass filter, with setting the gain ( ) at low frequencies and the capacitor maximizing the gain ( ) at high frequencies.
Example: Determine values for and that will yield the operating point shown in the circuit in figure 5.10 ( ) under the assumption that:
- Repeat both calculations for .
For , remains unchanged.
Figure 5.10: Example transistor circuit with a single bias transistor.
- If , determine an expression for the base-biasing resistor that will result in a DC operating point for the circuit in figure 5.11.
Now . The voltage drop across the bias resistor is . Charge conservation gives
The voltage at the base is . Neglecting the voltage drop gives
EliminOAating gives . Using gives
Substituting equation 5.35 into equation 5.33 gives
The voltage drop across the collector resistor is
- If the circuit is built with the just found, what will be the operating point if ?
If and , .
Substitution into equation 5.36 gives
Figure 5.11: Example transistor circuit with two bias resistors.