Multistage amplifier

Very often the voltage gain, power amplification or frequency response available from a single stage of amplification is not enough to drive the load or to build up a strong enough signal. Two or more additional stages are then connected one after another — cascaded — to achieve greater voltage or current amplification, or both. In practice an amplifier is almost always multistage: a radio receiver, for example, may use six or more stages. Any circuit containing more than two stages of amplification is called a multistage amplifier, and its basic purpose is to increase the overall gain. This topic is a standing favourite in the II PUC board exam, so it is worth understanding the gain arithmetic and the three coupling methods in detail.

Multistage amplifiers fall into two categories:

• Compound amplifiers — the stages may differ from one another (one may be common-base, another common-collector) and different types of inter-stage coupling may be used.
• Cascaded amplifiers — every stage, and the coupling between stages, is identical: identical single-stage amplifiers connected in cascade, with the output of one stage feeding the input of the next.

Need for cascading: because the gain of a single stage is not sufficient for practical applications, two or more stages are cascaded to build up a greater signal (Figure 3.6.1).

Overall gain of a multistage amplifier

The overall voltage gain of a multistage amplifier is the product of the individual stage gains. When gains are expressed in decibels, the overall gain is instead the sum of the individual decibel gains — which is one reason gain is so often quoted in dB. For the three-stage example of Figure 3.6.2, with stage gains 1000, 100 and 1000, the overall gain is $1000 \times 100 \times 1000 = 10^8$, which on the dB scale is $20\log_{10}(10^8) = 20 \times 8 = 160\ \text{dB}$.

Types of coupling in amplifiers

Coupling transfers the output of one stage to the input of the next. Three types are used:

1. Direct coupled amplifier (Figure 3.6.3) — the output of one stage is connected directly to the input of the next, with no coupling components. It is used where the load must be connected directly to the output (headphones, loudspeakers), where the DC output current does not disturb the load, and it is suitable for uniform amplification of DC and very-low-frequency AC signals.

2. RC coupled amplifier (Figure 3.6.4) — the output of one stage is connected to the input of the next through a resistor and a capacitor. It provides higher voltage gain and higher overall amplification than the other methods, and because it uses no coils or transformers there are no magnetic fields to pick up undesirable signals, so non-linear distortion is minimum. It is used for amplifying audio-frequency (AF) signals.

3. Transformer coupled amplifier (Figure 3.6.5) — the output of one stage is coupled to the next through a transformer. Because a transformer is frequency-sensitive its frequency response is poor and its frequency range is limited; it is costly and bulky (heavy iron core) and tends to introduce a humming sound because its coils can pick up stray signals. Transformer coupling is normally used only in the last stage, where the aim is to maximise power transfer by impedance matching to a low-impedance load — for example a loudspeaker whose impedance is only 4 Ω to 16 Ω, against a transistor stage output impedance of several hundred ohms.

Two-stage R-C coupled transistor amplifier

The R-C coupled amplifier is the most popular type because it gives excellent audio fidelity over a wide frequency range. Figure 3.6.6 shows a two-stage RC-coupled amplifier in CE configuration with voltage-divider bias. The coupling capacitor Cc joins the collector of the first stage to the base of the second; C1 couples the input signal into the first base and C2 couples the signal out of the last collector; R1, R2, RC and RE are the bias resistors, with RE providing stabilisation and CE bypassing AC across RE. Because each stage is coupled to the next by a series coupling capacitor followed by a shunt resistor, the circuit is called a resistance–capacitance coupled amplifier.

Working: an AC signal applied to the first stage is amplified and appears at its output with a 180° phase reversal. Cc feeds this signal to the second stage while blocking the DC bias of one stage from affecting the other. The second stage amplifies again with a further 180° reversal, so the final output is in phase with the input (the signal has been reversed twice). The overall voltage gain is the product of the two stage gains.

Frequency response (Figure 3.6.7): the voltage gain is small at low frequencies and at high frequencies and stays almost constant in the mid-frequency band. At low frequencies Cc offers a high reactance and passes only a small part of the signal, reducing the gain; at high frequencies Cc offers a low reactance and acts nearly as a short, increasing the loading of the next stage and reducing the gain. In the mid-band these two effects cancel and the gain stays constant. The bandwidth is the difference between the upper and lower cut-off frequencies, measured at the 0.707 (3 dB) points.

Advantages: cheap (only resistors and capacitors); good frequency response with uniform gain, so frequency distortion is low; minimum amplitude distortion (class-A operation, no coils/transformers); high overall gain compared with other couplings.

Disadvantages: the gain is comparatively small once loading of successive stages is taken into account; impedance matching is poor (high output impedance, low input impedance), so it cannot be used in the final stage against a loudspeaker; it becomes noisy with age and in moist climates; it cannot amplify DC or very-low-frequency signals; and power consumption is high because of the many resistors.

Applications: the initial (voltage-amplifier) stages of audio small-signal amplifiers such as radio receivers, TV receivers and public-address systems, because of its good frequency response.

Direct coupled transistor amplifier …