The pair of power MOSFETs in our amplifier (fig 9.1) which provide the large output current are said to operate in a push-pull manner. We've already looked briefly at this, but now we've examined MOSFETs in detail we can understand it a little more clearly. Figure 9.6 illustrates the output stage of the power amplifier. Q1 is an n-channel MOSFET (e.g. a 2SK135), Q2 is a p-channel MOSFET (e.g. a 2SJ50).
When we apply a positive gate voltage we allow current to pass through Q1 but not through Q2 so we ‘push’ current out to the load and produce a positive voltage across it. When we apply a negative gate voltage we allow current to pass through Q2 but not through Q1 so we ‘pull’ current out of the load and produce a negative voltage across it.
We can use the value of Vg as a control signal to get the current (and hence voltage) we require for the load (e.g. the loudspeaker). As a result, the current through the load is produced by a mixture ‘pushing’ and ‘pulling’ by the output devices — hence the term, push-pull for this kind of circuit. This process is illustrated in figure 9.7 where the circuit is used to play a sinewave into a resistive load.
Note that the current I2 shown here (and the currents through the p-channel device graph in figure 9.5) are negative — although both and appear to flow from ‘top to bottom’ in figure 9.6. This is because the two FETs are ‘different ways up’ in the amplifier. The n-channel one has its drain at the ‘top’ (i.e. most +ve) and the p-channel one has its source at the ‘top’. A conventional current flowing from top to bottom therefore corresponds to a +ve value for the n-channel device, but a -ve value for the p-channel device.
Summary
You should now know that a loudspeaker has an impedance which varies in a complicated way with signal frequency, but that it's overall behaviour can conveniently be summarised by a nominal impedance whose value is usually about 8 Ohms & assumed to be resistive. You should also know that decibels are a logarithmic measure of the ratio of two power levels. That units such as “dBm”, or “dBa” specify powers in decibels relative to a specific reference level.
You should now understand how an enhancement mode MOSFET works and that it's possible to make these — and other sorts of transistors — in complementary pairs. That complementary devices have their p-type & n-type parts ‘swapped over’ and behave in the same way, but with the signs of the voltages/currents reversed. That pairs of these devices can be used to make a push-pull arrangement which is useful as a current stage in a power amplifier to provide the large currents which may be needed. You should also understand that information is usually carried in terms of the signal's voltage pattern, so it's necessary for the power amp to monitor the voltage it's output current is producing and control it's output to obtain the desired result.
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit2 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.