We've seen in previous lectures how transistors can be used to amplify voltages. However, to provide the large powers required by a loudspeaker we also have to boost the signal current level. Power amplifiers usually work by first increasing the signal voltage to the required level, then passing the signal through a current amplifier section which provides all the extra current without altering the voltage. In the example shown in figure 9.1 the op-amp provides the voltage amplification and the pair of MOSFETs act as the ‘current stage’.
The way this works can be explain using the figurea 9.3a and 9.3b. The transistors, Q1 and act a bit like a pair of ‘taps’. An input control voltage, Vg, is used to adjust the currents flowing through the transistors. The current directed into (or drawn out of) the load is the difference between the two tap/transistor currents, I1 and I2. When the amplifier isn't being asked to produce an output voltage (i.e., when Vout is zero) we adjust the two transistors so that I1 = I2 and there's no current through (or voltage across) the load. To produce a positive load current & voltage we ‘turn up’ Q1 and ‘turn down’ Q2. To produce a negative load current/voltage we do the opposite — ‘turn down’ Q1 and ‘turn up’ Q2.
In a hi-fi amplifier-loudspeaker system information about the required sound pressure variations is carried by the pattern of voltage fluctuations not the current fluctuations. This wouldn't matter if the speaker was perfectly resistive since then the current would always be proportional to the voltage. However, as we've already seen, the voltage/current relationship for a speaker is quite complex. The amplifier therefore has to monitor the load voltage it's output current is producing and adjust the transistors to get the ‘correct’ output voltage pattern. This means the system has to include a control loop or feedback loop, which compares the output it's actually producing with what's required. The amplifier then provides whatever current pattern may be needed to achieve the correct signal voltage pattern & play the music.
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.