Section F2: Classes of Amplifiers

As mentioned in the introduction, an important role of the final stage of an amplification system is to ensure a low output resistance so that the output signal may be delivered to a low resistance load without loss of gain. Many times, the gain stages provide the output stage with signals that are large enough that the small-signal transistor model no longer applies (or must be used with caution). However, it is still of paramount importance that linearity be preserved – that is, signal distortion must be kept to an absolute minimum. Another output stage requirement is that it delivers the required power to the load efficiently. This means that the power dissipated in output stage transistors must be kept to an absolute minimum, both to ensure maximum power delivered to the load and, probably more importantly, to ensure that the transistor temperature remain below specified levels. Although MOS power transistors (to be discussed later this semester) offer significant advantages over BJTs such as the reduction or elimination of certain breakdown mechanisms found in BJTs, the reduction of the large drive currents required for BJT power amplifiers and a generally higher speed of operation, we will not address the MOSFET implementation this semester due to increased the device complexity required for high power applications.

Amplification stages are classified according to the characteristics of the collector (output) current waveform with an applied input. These classes are defined as Class A, Class B, Class AB and Class C. We will be investigating each of the classifications in the remainder of this section of our studies and will be concentrating on the discrete-circuit BJT implementation. The remainder of this discussion will involve a brief introduction into each class of amplifier, with subsequent discussions investigating each in greater detail.

Class A Operation

The Class A output stage reproduces the input signal in its entirety as shown in the figure to the right (Figure 8.1 in your text). The transistor of a Class A amplifier conducts for the entire cycle of the input signal or, as your author states, the collector current is non-zero 100% of the

time (except for the instantaneous zero crossings). The output, ic(t), is an amplified reproduction of the applied input ib(t). The letter designations (a) through (e) in the figure serve to represent the same instants in time on the ib(t) and ic(t) curves.

Class A operation may be implemented with a single transistor and is what we studied for BJT amplifiers in Sections C and D last semester (chapters 4 and 5 of your text). Class A operation will be introduced for FET amplifiers in Section J (chapter 6) of our studies. In the figure above, the bias point is shown in the middle of the ac load line for maximum possible output swing. This is not absolutely necessary for Class A operation, the only requirement is that ICQ be greater than the amplitude of the signal current to avoid nonlinear distortion. As a point of future discussion, remember that the maximum power dissipated by the transistor is equal to ICQVCEQ.

Class B Operation

Class B operation requires the use of two transistor amplifiers to produce a complete output waveform. One amplifier is used to amplify the positive half cycle of the input signal while the second is used to amplify the negative half-cycle. Each transistor is biased at a zero quiescent current (ICQ=0) instead of in the active region as in a Class A stage. The advantage of this configuration is that each transistor dissipates zero, or almost zero, power in the quiescent condition. The Class B configuration is also known as push- pull or complementary symmetry, and it is extremely important that the two transistors are perfectly matched so that the positive and negative portions of the input are amplified by the same amount.

A representation of the combined characteristic curves for a Class B stage is shown below (Figure 8.2 in your text). Note that the orientation of the curves is such that the ac load line may be presented continuously. Also note that the current designations in the figure below have been slightly changed from Figure 8.2. In your text, the author refers to the currents of transistor 1 with the subscript Q1 and the currents of transistor 2 with the subscript Q2. This does not mean quiescent conditions, but is a common means of identifying transistors. As we have seen (and will see much more), multiple transistor circuits identify individual transistors by the letter Q and sequential numbers, letters or symbols. I have attempted to avoid any possible misunderstandings and have labeled currents simply with ‘1’ or ‘2’ as appropriate. In this figure, the letter designations are intended to convey a time relationship between input and output – when transistor 1 is on, the input is ib1(t) and the output is ic1(t) for the time sequence (a), (b), and (c). Similarly, when transistor 2 is on, the input is ib2(t) and the output is ic2(t) for the time sequence (d), (e), and (f).

A significant disadvantage of Class B operation is related to the advantage mentioned above. Since the transistors are biased at ICQ=0, the operating range includes the nonlinear cutoff region. This results in some output signal distortion, known as crossover distortion, as illustrated in Figure 8.3 and to the right. As may be observed, the output resembles the input sinusoid, but is distorted near the collector current zero crossings.

Class AB Operation

In the introductory comments, I made mention of the two requirements for an output amplification stage – linearity and efficiency. Well, from the above discussions, we can see that the Class A configuration offers the better linearity (smaller distortion), but has pretty lousy efficiency (power dissipation). Conversely, the Class B stage does well with efficiency, but has rotten linearity. So…enter the Class AB configuration!

Class AB operation still uses two transistor amplifiers, one for the positive going portion of the input and one for the negative going input, but the bias of the individual transistors is between the extremes of Class A and Class B. In this strategy, each transistor is biased at a Q-point that is slightly above the cutoff region. Biasing each transistor in this fashion does two things:

* the Q-point remains in the linear region of the characteristic curves, which avoids the nonlinear distortion of the cutoff region; and * each transistor conducts for an interval slightly longer than a half-cycle.

The output currents from the two transistors are combined at the load, just like for the Class B (push-pull) configuration. The difference is that now, during the zero crossing interval, both transistors are conducting and the crossover distortion is reduced or eliminated.

The Class AB configuration is a compromise – the efficiency of the Class A and the linearity of the Class B stage have been significantly (although not perfectly) improved.

Class C Operation

For completeness, we will briefly introduce the Class C amplifier. These amplifiers are usually employed in RF (radio-frequency) power amplification and are capable of providing large amounts of power, but are somewhat application specific. Briefly, a transistor in a Class C stage is biased such that it conducts for an interval of less than a half-cycle of the input. The result is a waveform that pulsates periodically with the period of the input signal. If the input is a sinusoid, the output is a series of “blips” that contain the frequency of the input as the fundamental, plus higher frequency harmonics. The output may then be passed through an LC (inductor-capacitor) circuit tuned to the frequency of the input sinusoid. This tuned circuit acts as a bandpass filter and yields a sinusoid at the output that is approximately the same frequency as the input (the narrower the passband, the closer the approximation).

Power amplifier circuits contain transistors that are capable of handling high power (usually defined as greater than 1 Watt) and may possess significantly higher voltage and/or current ratings than conventional low- power transistors. In addition, protective circuits that limit current and techniques to dissipate heat are generally required. However, for our purposes, we are not going to focus on these specialized devices or techniques and will retain the small signal model for circuit analysis.