Transformer Coupled Class A Amplifier:

Class A Circuit – Instead of capacitor coupling, a Transformer Coupled Class A Amplifier may be used to ac couple amplifier stages while providing dc isolation between stages. The resistance of the transformer windings is normally very small, so that there is no effect on the transistor bias conditions.

Figure 18-1 shows a load resistance (R_L) transformer-coupled to a transistor collector. The low resistance of the transformer primary winding allows any desired level of (dc) collector current to flow, while the transformer core couples all variations in I_C to R_L via the secondary winding. This circuit is an emitter current bias circuit with voltage divider resistors R₁ and R₂ determining the transistor base voltage (V_B), and resistor R_E setting the emitter current level.

The circuit in Fig. 18-1 is referred to as a class-A amplifier, which is defined as one that has the Q-point (bias point) approximately at the center of the ac load line. This enables the circuit to produce maximum equal positive and negative changes in V_CE.

DC and AC Loads:

The total dc load for transistor Q₁ in the circuit in Fig. 18-1 is the sum of the emitter resistor (R_E) and the transformer primary winding resistance (R_py).

Consider the transformer illustrated in Fig. 18-2. N₁ is the number of turns on the primary winding, and N₂ is the number of secondary turns. The primary ac voltage and current are v₁ and i₁ and the secondary quantities are v₂ and i₂. The The (secondary) load resistance can be calculated as,

The ac load resistance measured at the transformer primary terminals (r_L) is calculated as,

From basic transformer theory,

These equations give,

Substituting for v₁ and i₁ in the equation for r_L,

The load resistance calculated in this way is termed the reflected load, or the referred load; meaning that R_L is reflected or referred from the transformer secondary to the primary as r_L. The total ac load at the transistor collector is the sum of the referred load and the transformer primary winding resistance,

Collector Voltage Swing:

The ac load line is shown in Fig. 18-5 to show the effect of an input signal. When the input causes I_B to increase from 50 μA (at I_BQ) to 90 μA, the current and voltage become I_C ≈ 9 mA and V_CE ≈ 1.6V, (point C on the ac load line). The changes are: ΔI_C = +4 mA, and ΔV_CE = -6.4 V. When the input causes I_B to decrease (from I_BQ) to 10 μA, I_C changes from 5 mA to 1 mA, and the V_CE changes from 8 V to 14.4 V, (point D). The current and voltages changes are now: ΔI_C = -4 mA and ΔV_CE = +6.4 V.

It is seen that an I_B change of ±40 μA produces a ±4 mA I_C change and a ±6.4 V change in V_CE. The V_CE variation appears at the primary winding of transformer T₁, and the I_C variation flows in the primary winding.

Note that, although V_CC = 13 V, the transistor V_CE can actually go to 16 V. This is due to the inductive effect of the transformer primary winding. The transistor used in this type of circuit should have a minimum breakdown voltage approximately equal to 2 V_CC.

Efficiency of a Class A Amplifier:

Power is delivered to an amplifier from the dc power supply. The amplifier converts the do power into ac power delivered in the load, (see Fig. 18-6). Some of the input power is dissipated in the transistor or in other components. This is wasted power. The efficiency (η) of a power amplifier is a measure of how good the amplifier is at converting the dc input (supply) power (P_i) into ac output power (P_o) dissipated in the load.

The do supply power is,

the case of a class A amplifier, I_ave = I_CQ.

Refer again to the class A circuit in Fig. 18-6, and assume that V_E ≪ V_CC. In this case, V_CEQ is approximately equal to V_CC, and the peak voltage developed across the transformer primary approaches ±V_CC if the transistor is driven to cutoff and saturation. Also, the peak current developed in the transformer windings approaches ±I_CQ. Thus, the maximum ac power delivered to the transformer primary can be calculated as,

Using the highest possible current and voltage levels, and assuming that the transformer is 100% efficient,

The maximum theoretical efficiency for a Class-A transformer-coupled power amplifier can now be determined as,

In a practical Class-A transformer-coupled power amplifier circuit, 50% efficiency is never approached. Any practical calculation of power amplifier efficiency must take the output transformer efficiency (η_t) into account.

A typical transformer efficiency might be 80%. There is also power dissipation in the transistor emitter resistor and in the bias circuit. The practical maximum efficiency for a Class-A power amplifier is usually around 25%. This means, for example, that 4 W of dc supply power must be provided to deliver 1 W of ac output power to the load.