Power Amplifier, Transformer Coupled Class A Amplifier, Class B Amplifier, Class C Amplifier

Power Amplifier

The ideal amplifier would supply its load with all of the power it receives from the dc power supply. Since each amplifier uses a portion of the power it takes from the dc power supply, it is practically impossible to achieve 100 percent efficiency (at this time).

The efficiency of an amplifier is the ratio of ac output power to dc input power, written as a percentage. By formula:

The lower the position of the Q-point on the dc load line, the higher the maximum theoretical efficiency of a given amplifier. Typical Q-point locations for class A, B, AB, and C amplifiers are shown in Figure 11.1 of the text.

AC Load Lines

The ac load line is a graph of all possible combinations of ic and vce for a given amplifier. Under normal circumstances, the ac and dc load lines for a given amplifier are not identical (see Figure 11.3 of the text).

Amplifier Compliance

An amplifier’s peak-to-peak output voltage is constrained by the output circuit by its compliance (PP). The following equations are used to determine the compliance for a particular amplifier.

PP = 2lCQrC and PP = 2VCEQ

These equations are developed as illustrated in Figure 11.1.

The compliance of an amplifier is determined by solving both PP equations and using the lower of the two results, as demonstrated in Example 11.1 of the text. Note the following:

When an amplifier has a value of PP = 2VCEQ, exceeding the value of PP results in saturation clipping.

When an amplifier has a value of PP = 2lCQrC, exceeding the value of PP results in cutoff clipping. However, the circuit will experience nonlinear distortion before the amplifier peak-to-peak output reaches the value of PP.

1. Transformer Coupled Class A Amplifier

A transformer-coupled class A amplifier is shown in Figure 11.2. The transformer is used to couple the amplifier output signal to its load.

With the exception of the fact that the value of VCEQ is intended to be as close as possible to the value of VCC, the dc biassing of the transformer-coupled class A amplifier is comparable to that of other amplifiers.

Plotting the ac load line of a transformer-coupled class A amplifier is demonstrated in Section 11.3.3 of the text. The following are typical characteristics for the transformer-coupled circuit:

VCEQ is very close to the value of VCC.

The maximum output voltage is very close to 2VCEQ and therefore, can approach the value of 2VCC.

The maximum theoretical efficiency of a transformer-coupled class A amplifier is 50%.

In practice, the transformer-coupled amplifier has a value of < 25%. The high theoretical value is a result of assuming that VCEQ = VCC and ignoring transformer (and other) circuit losses. The efficiency of a transformer-coupled circuit is calculated as shown in Example 11.7 of the text.

The transformer-coupled class A amplifier has the following advantages over the RC-coupled circuit:

Higher efficiency.

It is relatively simple to match the amplifier and load impedance using a transformer.

A tuned amplifier, or a circuit that offers a specific gain over a given range of operating frequencies, can be created easily from a transformer-coupled circuit.

2. Class B Amplifier

The class B amplifier is a two-transistor circuit that is designed to improve on the efficiency characteristics of class A amplifier. A class B amplifier is shown in Figure 11.3. The Q-point values for the circuit in Figure 11.3 are found using

where ICO is the collector cutoff current rating for the transistor.

The circuit shown in Figure 11.3 is a Complementary-symmetry Amplifier, or a Push-pull emitter follower. And The circuit contains one npn transistor (Q1) and one pnp transistor (Q2). The circuit Contains complementary transistors; that is, npn and pnp Transistors with Identical Characteristics

3. Class C Amplifier

The transistor in a class C amplifier conducts for less than 180° of the input cycle. A basic class C amplifier is illustrated in Figure 17.14.

The most important aspect of the dc operation of this Amplifier is that it is biased deeply into cutoff, meaning that VCEQ =~~=VCC and ICQ =~~= 0 A. If a negative supply is used to bias the base circuit, the value of VBB usually Fulfills the following relationship:

–VBB = 1 V – Vin(pk)

The ac operation of the class C Amplifier is based on the Characteristics of the Parallel-resonant tank circuit. If a single current pulse is applied to the tank circuit, the result is a Decaying Sinusoidal Waveform (as shown in Figure 17.43b of the text). The Waveform shown is a result of the Charge/discharge cycle of the Capacitor and Inductor in the tank circuit, and is Commonly referred to as the Flywheel effect.

We apply a current pulse during each full cycle Repeatedly to obtain a sine wave that does not decay. A class C Amplifier’s tank circuit receives the current pulse it requires to produce a full sine wave at the output at the peak of each positive Alternation of the input signal. Figure 17.44 in the text provides an Illustration of this idea. Keep in mind that T1, T2, and T3 are Reversed at the output compared to the input. This results from a 180° voltage phase shift produced by a Common-emitter Amplifier. It should be noted that a class C Amplifier has the same Bandwidth, Q, and QL Characteristics as any other tuned Discrete Amplifier.

One last Observation regarding the class C Amplifier. The tank circuit must be tuned to the input Signal’s Frequency, or to one of its Harmonics, for this Amplifier to function properly. As an Illustration, if the class C Amplifier was tuned to the third Harmonic of the input, the output would be three times the input Frequency. The class C Amplifier can therefore be Utilised as a Frequency Multiplier.