Davis W.A.Radio frequency circuit design.2001

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iout

Q 1 on

Q 2 on

FIGURE 9.3 The reconstituted waveform of a class B amplifier.

transistors is cut off when the total voltage is less than 0, only the positive half of the wave is amplified. The conduction angle is 180°. The term, class AB amplifier, is sometimes used to describe the case when the dc bias current is much smaller than the signal amplitude, IOC, but still greater than 0. In this case,

180° < conduction angle − 360°

9.2.1Complementary (npn/pnp) Class B Amplifier

Figure 9.4a shows a complementary type of class B amplifier. In this case transistor Q1 is biased so that it is in the active mode when the input voltage, vin > 0.7 and cut off when the input signal vin < 0.7. The other half of the signal

FIGURE 9.4 (a) The basic complementary class B amplifier, (b) class B amplifier with diode compensation to reduce crossover distortion, and (c) class B amplifier with a VBE multiplier to reduce crossover distortion.

In the middle where VX D 0 the VCO and VO can be forced to be equal by adjustment of the resistors R1 and R2,

In addition to reducing or eliminating the dead band zone, the compensation circuits in Fig. 9.4b and 9.4c also provide for temperature stability, since a change in the temperature changes the transistor VBE value. The compensation circuit and the power transistors vary in the same way with temperature, since they are physically close together.

Another aspect that deserves attention is the actual value of the current source, IQ. Since this supplies the base current for the npn output transistor Q1, IQ must be large enough to not “starve” Q1 when it is drawing the maximum current through its collector. This means that IQ ½ IC1/ˇ1.

9.2.3The Composite pnp Transistor

One of the primary problems in using this type of class B amplifier is the requirement for obtaining two equivalent complementary transistors. The problem fundamentally arises because of the greater mobility of electrons by over a 3 : 1 factor over that of holes in silicon. The symmetry of the gain in this circuit depends on the two output transistors having the same short circuit base to collector current gain, ˇ D ic/ib. When it is not possible to obtain a high ˇ pnp transistor, it is sometimes possible to use a composite transistor connection. A high-power npn transistor, Q1, is connected to a low-power low ˇ pnp transistor, Q2, as shown in Fig. 9.5. Normally the base–emitter junctions of the composite and single pnp transistor are forward biased so that the Shockley diode equation may be used to describe the bias currents. For Q2 in the composite circuit,

FIGURE 9.5 A composite connection for a pnp transistor.

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The collector current for Q1 in the composite circuit is the same as the collector current for the single pnp transistor:

The composite circuit has the polarity of a pnp transistor with the gain of an npn transistor.

9.2.4Small Signal Analysis

The three fundamental parameters that characterize an amplifier are its voltage gain, Av, input resistance, Rin, and output resistance, Rout. In the circuit shown in Fig. 9.4a, neither Q1 nor Q2 are on simultaneously. If Q1 is on, Q2 is an open circuit and need not be considered as part of the ac analysis. A small signal hybrid model (Fig. 9.6) for a bipolar transistor consists of a base resistance, rb, base–emitter resistance, r , collector–emitter resistance, ro, transconductance, gm, and short circuit current gain ˇ D gmr . There are in addition high-frequency effects caused by reactive parasitic elements within the device. Since the voltage gain of an emitter follower is ³ 1, the voltage gain of the Q3 and Q1 combination is

The effective load resistance RL eff seen by the first transistor, Q3, is the same as the input resistance of the emitter follower circuit Q1. Circuit analysis of the low-frequency transistor hybrid model would give (Appendix F)

FIGURE 9.6 The small signal hybrid model of a bipolar transistor.

The low-frequency input resistance to the actual class B amplifier is given by RL eff in Eq. (9.17), and the output resistance is

Thus the input resistance is high and the output resistance is low for a class B amplifier, which enables it to drive a low-impedance load with high efficiency.

9.2.5All npn Class B Amplifier

The complementary class B amplifier shown in Fig. 9.4 needs to have symmetrical npn and pnp devices. In addition this circuit also requires complementary power supplies. These two problems can be alleviated by using the totem pole or all npn transistor class B amplifier. This circuit requires only one power supply, and it has identical npn transistors that amplify both the positive and negative halves of the signal. However, it requires that the two transistors operate with an input phase differential of 180°. This circuit is illustrated in Fig. 9.7. Clearly, the cost of the all npn transistor amplifier is the added requirement of two centertapped transformers. These are necessary to obtain 180° phase difference between Q1 and Q2. The center tapped transformer also provides dc isolation for the load. When the input voltage is positive, Q1 is on and Q2 is off. When the input voltage is negative, the input transformer induces a positive voltage at the “undotted” secondary winding which turns Q2 on. The output of Q2 will induce on the output transformer a positive voltage on the “undotted” terminal and a negative voltage on the “dotted” terminal. The negative input voltage swing is thus replicated as a negative voltage swing at the output. The transformer turns

Q 1

R L

+

FIGURE 9.7 The all npn class B amplifier.

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is supplied through a resistor and max D 50% when the bias to the collector is supplied through an RF choke.

9.3THE CLASS C AMPLIFIER

The class C amplifier is useful for providing a high-power continuous wave (CW) or frequency modulation (FM) output. When it is used in amplitude modulation schemes, the output variation is done by varying the bias supply. There are several characteristics that distinguish the class C amplifier from the class A or B amplifiers. First of all it is biased so that the transistor conduction angle is <180°. Consequently the class C amplifier is clearly nonlinear in that it does not directly replicate the input signal like the class A and B amplifiers do (at least in principle). The class A amplifier requires one transistor, the class B amplifier requires two transistors, and the class C amplifier uses one transistor. Topologically it looks similar to the class A except for the dc bias levels. It was noted that in the class B amplifier, an output filter is used optionally to help clean up the output signal. In the class C amplifier, such a tuned output is necessary in order to recover the sine wave. Finally, class C operation is capable of higher efficiency than either of the previous two classes, so for the appropriate signal types they become very attractive as power amplifiers.

The class C amplifier shown in Fig. 9.9 gives the output circuit for a power bipolar transistor (BJT) with the required tuned circuit. An N-channel enhance- ment-mode metal oxide semiconductor field effect transistor (MOSFET) can be used in place of the BJT. The Q of the tuned circuit will determine the bandwidth of the amplifier. The large inductance RF coil in the collector voltage supply ensures that only dc current flows there. During that part of the input cycle when the transistor is on, the bias supply current flows through the transistor and the output voltage is approximately 90% of VCC. When the transistor is off, the supply current flows into the blocking capacitor. The current waveform at the

FIGURE 9.9 A simple class C amplifier where VBB determines the conduction angle.