Multistage amplifiers and Negative feedback.

Multistage amplifiers

Usually an AC amplifier is required to have a gain considerably higher than that obtained with a single transistor stage. Amplifier stages can be cascaded to give the required gain, the overall gain being given by:

Design of multistage amplifiers needs some care if problems are not to be encountered with oscillations. Many a multistage amplifier has turned into a multistage oscillator.

A multistage amplifier is shown. Transistor TR3 causes quite major current variations in the supply current, and if supply regulation is poor or the supply leads have significant inductance the supply voltage varies in sympathy.

These variations are fed back to the first stage where they are treated as signal, so are amplified. If conditions are right, oscillation of the amplifier occurs.

To overcome this problem, supply decoupling for the first few stages is usually included. Resistor R 13 and capacitor C8 provide supply decoupling for TR 1 in the above multistage amplifier.

The second problem with multistage amplifiers is oscillations induced by stray capacitance. This is a layout problem, and should not arise if early stages are kept clear of large amplitude signals in later stages, and interconnecting wires are kept short and tidy. Oscillations caused by stray capacitance are normally very high and if frequency of oscillation is above the desired amplifier bandwidth, a cure can be effected by deliberately reducing the high frequency gain.

One final point to note with multistage amplifiers is the problem of self-induced noise. Transistors produce white noise, partly due to irregularities in the electron flow (shot noise) and partly from thermal agitation of the electrons in the circuit impedances.

Transistor noise has two components; intrinsic noise which depends on emitter current and excess noise which depends on collector volts.

Any noise occurring in early stages of a multistage amplifier is amplified by successive stages, to the detriment of the amplifier performance. To reduce noise, early stages should be run at low currents and low voltages. Noise can also be reduced by the use of special low noise transistors (e.g. BC149).

It is unusual to find multistage amplifiers without overall feedback to determine gain (see below). It was shown earlier that variations in transistor characteristics cause wide variations in the gains of apparently identical single-stage amplifiers.

In multistage amplifiers, the effect is multiplied. For example, if an amplifier is constructed of three stages, each of which can have a gain variation of 5 to I, the total variation in gain could be as much as 125 to I.

Multistage amplifiers are normally constructed such that the minimum possible gain (calculated with the worst transistor characteristics) is more than adequate. The overall gain is then determined by negative feedback, giving an amplifier whose performance is consistent regardless of the transistors used in the circuit.

Negative feedback

Variations in transistor characteristics give large differences between gains of apparently identical amplifiers. Negative feedback is widely used to produce amplifiers with predictable gains and low distortion.

In the circuit below is a high gain amplifier A. Its output voltage is attenuated by B and subtracted from the input voltage. Simple analysis shows that:

If, for example, A is 100 and B is 0.1, the overall gain is 9.1. If A is doubled to 200, and B remains at 0.1, the overall gain is 9.5, a negligible change. If A is very large the overall gain can effectively be considered 1/B.

Negative feedback is also useful in reducing distortion, although this may not be immediately obvious. If an amplifier has N% distortion in its open-loop state, the distortion with negative feedback is:

The term (I+ AB) is sometimes called the gain reduction factor, and the term AB the open-loop gain. The overall gain N(l + AB) is called the closed-loop gain.

Audio amplifiers

Audio amplifiers are designed to be used over the range around 30 Hz to 20 kHz. Over this range gain has to be constant if reproduction is not to exaggerate one particular range of frequencies. Further, the human ear is quite sensitive to distortion, so the output stage has to be designed for minimum distortion.

Classically, audio amplifiers have been designed in two blocks. A pre-amplifier (or control) stage is used to amplify the input signal to a level of around I V. This stage also contains the user's volume and tone controls together with input and mode selection switches.

Second block is a power amplifier, used to deliver considerable power to the speakers. This usually has no user controls.

Most amplifiers now contain a pre-amplifier and power amplifier in a single case, but it is still convenient to deal with them as separate topics. The remainder of this section therefore discusses

pre-amplifier design, while the following section deals with power

amplifier design.

The input stage has to accept an input from a wide range of  sources and at levels from a few millivolts for a magnetic pick-up to several hundred millivolts for an input from a radio tuner. The mode selection switch must therefore not only select the input, but must also select a suitable gain.

Further complication is added by the response of a magnetic cartridge. This is not flat, but has a varying response defined by the RIAA (Record Industry Association of America) standard. The selected gain for a magnetic cartridge has to be 'equalised' to give a flat response.

A typical input stage is shown. Transistors TR 1 and TR2 form a simple two-stage amplifier, with DC stabilisation provided by resistor R3. The mode switch SWIa selects the required input while the ganged switch SW1b switches feedback components to give correct gain for each input signal. In the magnetic pick-up position, for example, components R6, C3, C4 give RIAA equalisation.

Transistors TR 1 and TR2 are specially chosen for their low noise characteristics, and care taken with the wiring to SW1 to obviate the possibility of interference from mains transformers or RF pick-up.

The input stage is usually followed by tone controls. These are typically some form of bass/treble lift/cut circuit. A typical passive circuit is shown over in (a), along with an active circuit using negative feedback in (c). These have the basic response of (b).

The human ear has a very nonlinear response, and at low volumes it is less sensitive to low frequencies. On some amplifiers a loudness control is used in place of a simple attenuator volume control. This provides progressive bass lift as volume is reduced, to compensate for the response of the ear. Hi-fi purists tend to decry the loudness control and prefer a simple volume control.