Class B power amplifier

Consider in detail the class B amplifier operation (Q₃ and Q ₅). In Fig. 5, the circuit diagram of the output section is shown. The transfer characteristic of the output section is shown in Fig. 6.

The transistors are operated from the power supplies of equal voltage magnitude, V_CC. The output signal, v₀ (t), will be obtained as a function of the input signal, v_I (t) = V_m sinωt. For v_I = 0, both Q₃ and Q₅ are OFF; i _C3 = i_C5 = 0, and as a result, v₀ = 0. As v_I increases positively, Q₅ remains OFF, and when v_in > V_BE(ON), Q₃ turns ON. During this interval, Q₃ operates as an emitter follower, where

Fig. 5 Output class B amplifier

Fig. 6 Class B amplifier transfer characteristic

The output voltage maximum is

The input voltage necessary to drive v ₀ to V_CC - V_{CE3( sat)} is given by

In most circuits, because v₁ cannot exceed V_CC, Q₃ never saturates since V_BE(ON) > V_CE(sat). As v₁ becomes negative, Q₃ remains OFF, and when v₁, drops below V_BE5(ON) = 0.7V, Q₅ turns ON. Transistor Q₅ now behaves as a p-n-p emitter follower:

And it occurs for an input voltage of

As seen from Fig. 6, there is the 1.4-V dead band in the center of the characteristic. Such transfer characteristic leads to a distorted output waveform. This is so called crossover distortion shown in Fig. 7.

Fig. 6 Illustrating how the dead band in the class В transfer characteristic results in crossover distortion

II. PROBLEMS AND INDIVIDUALS DATA

Using a CA 3096 transistor array, design an operational amplifier to obtain a voltage gain A_V = if V_EE = V_CC, V_BE1 = V_BE2 = V_BE3 = 0.7V, V_BE4 =V_BE5 = - 0.7V, V_T = 26mV, β ₁= β₂= β₃= 188, β₄ = β ₅= 45, and R_L= l0kΩ. The resistor values should be compatible with the monolithic IC fabrication technology, i.e. any resistor value must not exceed 50kΩ.

Required voltage gain = 275;

Power supply voltage V_EE=V_CC=12V;

The voltage gain of the designed operational amplifier must satisfy the following inequality:

a. Determine R₁,R₂, R₃, R₄, and R ₅;

b. Calculate A_V1, A_V2, and A_V;

c. Compute R_id, R_ic, R_o3, and R_o5.

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