Capacitor Coupled Common Base Amplifier:

A practical Capacitor Coupled Common Base Amplifier (using a plus/minus supply) is shown in Fig. 12-39. [Note the polarity of the input coupling capacitor (C₁).] The CB circuit is analysed already, where it is found to have a very low input impedance, as well as a voltage gain and output impedance similar to a CE circuit, Because the CB circuit has no phase inversion between the input and output terminals, there is no Miller-effect amplification of the collector-base capacitance. Consequently, a CB circuit is capable of operating at much higher frequencies than a CE circuit.

Design procedure for a CB circuit is exactly the same as that for a CE circuit. Resistor calculation is performed just as if a CE circuit were being designed. The output coupling capacitor (C₂ in Fig. 12-39) is determined in the usual way. Because of the very low input impedance of a CB circuit, the input coupling capacitor (C₁) is the largest capacitor in the circuit. So, C₁ determines the circuit low cutoff frequency, and its capacitance is calculated like an emitter bypass capacitor in a CE circuit.

Cascode Amplifier:

The very low input impedance of a CB circuit (typically 25 Ω) is a major disadvantage. To increase the circuit input impedance while retaining its high frequency performance, the cascode amplifier (Fig. 12-40) uses a CE input stage driving a CB output stage Transistor Q₁ and its associated components operates as a CE input stage, while the circuit of Q₂ functions as a CB output stage.

The circuit dc voltage and current levels are illustrated in Fig. 12­-41. It is seen that I_E1 is set by V_B1 and R_E. Collector current I_C1 approximately equals I_E1 and I_E2 is the same current as I_C1. So, I_C2 approximately equals I_E1. The current levels remain constant so long as I_CE1 and I_CE2 are large enough for transistor operation.

A positive-going ac signal at the base of Q₁ produces an increase in I_E1 and this results in an I_C2 increase. Thus, the voltage drop across R_C is increased, producing a decrease in the output voltage. So, there is an ac phase inversion between the input and output.

The input impedance at the emitter of Q₂ constitutes the load for the collector of Q₁. Therefore, the voltage gain from the circuit input to Q₁ collector is,

With an input stage gain of -1, there is no significant Miller effect at the circuit input.

The voltage gain produced by the Q₂ CB circuit is,

Converting to CE parameters, the overall voltage gain for the cascode amplifier is the same as that for a CE circuit;

Design Calculations:

The design procedure for a cascode amplifier is easily derived from the CE design process. The voltage across R_E should typically be 5 V, and the V_CE for each transistor should be a minimum of 3 V. The emitter voltage for Q₂ is,

The transistor base voltages are,

When the voltage and current levels are selected, the resistor values are easily calculated.

The base bypass capacitor (C₂) in Fig. 12-40 should have an impedance equal to one tenth of the h_ie of Q₂. This ensures that the impedance looking into Q₂ emitter terminal is not significantly affected by R₁ and R₂. Also, that no portion of the input signal (at Q₂ base) is developed at Q₁ base.

Differential Amplifier as a High-Frequency Amplifier:

The differential amplifier discussed already can have the same high-frequency performance as a CB circuit. The differential amplifier circuit shown in Fig. 12-43, has zero voltage gain from Q₁ base to Q₁ collector, because there is no collector resistor. Consequently, there is no Miller effect on the input capacitance.

Also, the amplifier input is applied (via Q₁) to the emitter of Q₂, as in the case of a CB circuit. So, there is no Miller effect associated with Q₂. The input impedance (at the base of Q₁) is reasonably high, like a CE circuit and unlike the case of a Capacitor Coupled Common Base Amplifier.