Amplifier Basics

A typical (e.g. public address) amplifier amplifies the small AC signal produced by an input transducer (e.g. microphone) up to the much higher power level required to drive an output transducer (e.g. speaker).

The power gain A_P is the ratio of output power to input power and as it is usually a large number we often express it in decibels (dB) to "compress" the range. Another reason for expressing gains in dB is that the human ear has a logarithmic response insofar as perceived sound levels are concerned. For example, graphic equaliser boost and cut levels are graduated in dB. A one decibel change in sound pressure level is generally accepted as the minimum change detectable by a keen human ear. Interestingly, doubling the power output of an amplifier only produces a 3dB increase in perceived sound pressure level.

where A_P(dB) = 10 log₁₀(A_P)

Since power is also proportional to the square of the voltage, voltage gain may also be expressed in dB

where A_v(dB) = 20 log₁₀(A_v),  A_v being the voltage gain V_out/V_in.

A voltage gain of 10 corresponds to 20dB, 100 to 40dB, 1000 to 60dB, and so on.

Since the amplifier provides power gain, and the input transducer provides very little power, the increased power in the load must largely come from the DC power supply of the amplifier. Also, as the amplifier is itself not 100% efficient, some of the power (typically about 30%) provided by the supply to the amplifier is dissipated as heat. High power amplifiers have large "heatsinks" (more correctly described as heat exchangers) to help dissipate this heat.

A simplified diagram (no controls,etc shown) or "model" of an amplifier system is shown below.
With reference to the circuit above, R_in is the input resistance (seen by the microphone) looking into the amplifier input terminals. Note that the microphone is connected by a shielded or co-axial cable to minimise the pickup of unwanted signals (e.g. 50Hz mains "hum"). A_v  is the "open circuit output" voltage gain V_o/V_in and R_o is the output resistance (seen by the speaker, looking into the output terminals). R_L is the load resistance (e.g. 8W speaker). Note the convention of showing the input on the left and the output on the right.

The amplifier type shown above is a voltage amplifier, or VCVS (voltage controlled voltage source). A common convention is to give controlled or dependent sources a diamond shape, as shown. For such an amplifier, we try to make R_in >> R_s where R_s is the internal resistance of the source (e.g. 500W for a typical microphone). We also try to make R_o << R_{L .}

Typically, R_in may be 200kW, and R_o may be 0.1W

Under the above conditions, the overall voltage gain V_o/V_s will approach the value A_v but will always be somewhat less than this value due to voltage divider effects between R_s and R_in at the input, and R_o and R_L at the output. The ideal values of R_in and R_o are infinity and zero respectively. With these  ideal values, voltage transfer into and out of the amplifier are maximised, and  V_o/V_s will equal A_v.

We can gain a feel for the value of overall voltage gain A_v required by  considering the following example.

Suppose we require an average or steady state power of 100W in the 8W load. It follows that, from the relationship that power P = V²/R

Solving this gives us V_ORMS = 28.28V

A typical high quality microphone produces a signal of around 200mV_RMS , so the overall value of voltage gain A_v ( = V_o/V_in) required will be

Note that A_v has no units, as it is the ratio of two voltages. Converting this numerical gain to decibels gives A_v = 20log141,000 = 103dB.

Usually, this level of amplification is achieved by using several amplifier stages, often employing Operational Amplifier integrated circuits in the first stages immediately following the low level microphone input.

Such low level or "small signal" Operational Amplifier circuits form the major content of this module.

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