One disadvantage of series fed class a power amplifier is low efficiency. Another disadvantage is that a large number of DC current components pass through the load R_{L}Sometimes we only need the amplified AC component on the load, not the DC component (such as loudspeaker). At this time, transformer coupled class a power amplifier can be used.

Transformer coupled class a power amplifier uses transformer coupling to output to the load. The load can only receive the amplified AC signal without DC component. Another advantage is that the maximum power efficiency can reach 50%. A basic transformer coupled class a power amplifier circuit is shown in the figure below

Figure 5-03.01

## 1. DC analysis

When there is no input voltage V_{i}At this time, the current in the transformer is not alternating, the resistance of the coil winding is very small (generally from a few tenths of a ohm to several ounces), the secondary coil can not get any power, and the load R_{L}No power output at all. Because the DC resistance of transformer coil is very small, the slope of DC load line is very large, which is approximately vertical. We can regard it as a vertical straight line, and its intersection point with X axis is approximately v_{CC}As shown in the figure below:

Figure 5-03.02

Then the DC parameter I is calculated_{BQ}I_{BQ}After the DC load line with this I_{BQ}The intersection point of the corresponding output curve is the static working point Q.

## 2. Communication analysis

When the input voltage is v_{i}When it is sinusoidal small signal voltage, the situation is completely different. Sinusoidal alternating V_{i}Will make the collector current I_{C}At this time, the transformer will start to work. The secondary coil of the transformer will sense a sinusoidal voltage, and then load R_{L}The equivalent resistance value mapped to transformer primary is as follows:

As long as there is a sinusoidal input v_{i}，R_{L}R will be mapped in the primary transformer_{L}And this resistance does not change with V_{i}Frequency and amplitude of the change and change, always presents a fixed value. Therefore, when there is an AC input signal v_{i}The collector of BJT transistor is equivalent to a constant resistance R_{L}‘。 The characteristic of this resistor is that as long as there is an AC input signal v_{i}When there is no AC input signal v_{i}It does not exist (the resistance becomes approximately 0).

Therefore, in the presence of AC input signal v_{i}In this case, because the collector has more resistance R_{L}We need to draw a new load line, as shown in the following figure:

Figure 5-03.03

General literature or model books will be called: AC load line. However, this term is often confusing to beginners. Strictly speaking, it should be called: r when there is communication input_{L}Mapping to fixed resistance R_{L}The load line is a little awkward, but it helps to clarify the concept.

Now we have a common emitter circuit with only one emitter_{L}, so the slope is – 1 / R_{L}‘。 According to the theory of transformer, when AC sine input v_{i}At the moment of passing through point 0, the potential induced by the primary coil of the transformer is exactly 0. Therefore, the instantaneous I_{B}、i_{C}、v_{CE}It is exactly equal to the I calculated in the previous DC analysis_{B}、I_{C}、V_{CE}Therefore, the AC load line will intersect the static operating point Q. Using slope and q-point, we can draw the AC load line by drawing, as shown in figure 5-03.03.

It should be noted that the primary coil of the transformer will produce an induced potential, which is superimposed with V_{CC}After that, the intersection point of AC load line and X axis is greater than v_{CC}。 Therefore, it is necessary to verify that the fluctuating voltage does not exceed the tolerance of the CE terminal of the transistor

## 3. Power analysis

**Input power:**

At that time, when the input signal is 0, the power consumed by the amplifier circuit is the bias power to maintain the static working point

The current I obtained from the power supply when there is an input signal_{C}Because of the symmetry of the sinusoidal signal, the average power obtained from the power supply remains unchanged and can still be expressed by the above formula.

**Output power:**

When there is an input signal, the output power is the load R_{L}In general, the RMS is used to represent the AC voltage and AC current. The output power is expressed as follows:

## 4. Efficiency analysis

In the most ideal case, the load can get the maximum voltage fluctuation range, and the static working point is in the middle of the output curve_{CC}In the best case, the AC between the AC load line and the x-axis is 2V_{CC}The intersection point with y axis is 2I_{CQ}At this time, the full range of AC load line can be fully utilized without distortion of output waveform, as shown in the figure below.

Figure 5-03.04

In this ideal case, the RMS of output voltage and output current are respectively:

Therefore, the theoretical maximum efficiency is as follows:

This is only the theoretical maximum efficiency of the transformer coupled class a power amplifier in the most ideal situation. In practical application, it is difficult to make the static working point exactly in the center of the AC load line, so the actual transformer coupled class a power amplifier’s working efficiency will be less than 50%.

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