amplifieraudioclass-d
I've been wondering what could possibly limit the use of class-D amplifier to audio range frequencies. When I look at some class-d amplifier specs, it seems that a lot of them can't reach great linearity at the end of this range.
Why can't we use them above that limit ?
Is the modulation method part of the answer ?
Have you looked at the Molex PDJack - PDJack™ Powered Device Connector Module It is also available as part of a development kit that has the Silvertel components integrated. Might be worth taking a look.
It runs in class A as both output devices conduct for the entire cycle of the signal.
It can provide twice the quiescent current into the load because as Q3 increases in current, Q2 reduces. The opposite occurs for negative going signals at the input.
The quiescent current is mainly defined by the value of R4 and the gain of the transistors. Approximately half of the current through R4 goes into the base of Q2, the other half flows through Q1 into the base of Q3 with some getting diverted into R5.
The quiescent current will vary with the gain of the transistors although R5 and R6 will help stabilize it.
That is not a very good version - it has no bootstrapping so will be limited in output excursion and it depends upon the Hfe of the two output transistors being matched as well as having to be adjusted for the specific transistors.
One of the earliest well-known implementations of this configuration for audio was in Wireless World magazine in 1969.
Note that compared to the poster's circuit that the resistor feeding the collector of the driver transistor is split into 2 with a capacitor driving the junction from the output. This bootstrapping allows the output to swing to the positive rail. In the poster's circuit the current through R4 reduces as the output voltage goes positive starving the output transistor of base current.
In non-audio use a similar configuration is used as the output of TTL gates.
Best Answer
Another consideration that leads to class D amplifiers not being used at high frequencies is the increasing proportion of the total time consumed by the (lossy) switching operation as frequency increases.
There is a limit to how quickly switching can be accomplished: if it can be done in 100ns, then 1/250 part of a 20kHz cycle is consumed by switching; one still has a reasonable number of graduations in the pulse width and thus the possible amplitude of an eventual 20kHz tone, so relatively low-order output filters can be used. At 200kHz, with switching alone taking 1/25th part of the cycle, amplitude resolution would be much reduced and higher-order filters would be required to limit the harmonic content of the output.
Perhaps switching could be accomplished in 10 rather than 100ns, but probably not much less than that, especially if dead time is provided so as to limit switching losses. At higher frequency, switching must (self-evidently) occur more often, and given that it takes a certain minimum time, then at some point the amplifier will need to switch often enough that it no longer has the benefit of being in the low-loss, purely on or off state for long enough to justify the class D topology. As frequency and power output increase, the efficiency benefit of class D over class AB becomes increasingly marginal.