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特意把相关部分洋文抄了下来，能看懂这部分洋文应该也能了解差不多了。（期待有人详细解说一下）
DC Control Amplifier
The purpose of this to additional input stage is to provide an accurate, low drift, dc gain path to the main output this is independent of the AC gain path and its poor DC characteristics.
In the original version of this amplifier, expensive precision matched NPN and PNP dual transistors were used in the two current mirrors, but no DC control amplifier was used.
It was incorrectly assumed that precise matching of the transistors in each mirror would result in very low output offset voltage as as long as the input buffer had reasonably low input offset voltage as well.
As it happens, thia is not the case with a current feedback amplifier. Any mismatch between the two current mirrors results in a finite amount of bias current appearing at the output terminal of input buffer,
which must flow through feedback resistor R8 to the output. It cannot flow through R6 and R7 to ground, because the current in these resistor is set only by the voltage appearing at the output of the inpt buffer.
The output offset voltage,without the DC control amplifier is thus:
Voos=Viso(A1)[1+R7/R8][1+R8/(R7+R7)
Normally, Vios(A1)can be made quite small by using a low offset op amp. Unfortunately, the out terminal bies current,
Ibias can be as large as 100uA under static conditions and even larger if a thermal gradient exists between the two mirrors on the power amplifier driver board.
This can easily lead to an output offset in excess of 100mV,which changes as the amplifier warms up.
A large offset like this is likely to cause an audible click when the relay that connects the loudspeakers to the amplifier is energized, and is generally undesirable.
The solution to these problems is a low frequency servo loop the controls the DC output voltage, independently of any low frequency current or voltage fluctuations in the main current feedback gain path.
this is facilitated by the use of a second low power precision OP amp, A2,that is configures as an integrator with very low crossover frequency (less then 5Hz).
the low crossover frequency ensures that the integrator will not have any effect on the performance of the overall amplifier in the audio band.
Voltage feedback is applied from the main output back to the input of the integrator through resistors R10 and R11, which set the closed-loop DC gain.
This gain is made equal to that given by equation(1). Since A2 drives a resistor connected to ground, as shown in figure 2.
it behaves as an operational transconductance amplifier with the output current taken from its power supply terminals.
This compensating out current is then fed to the two common-base regulator transistors where it is summed with the signal current from the power supply terminals of A1.
The output current of A2 is thus forced to cancel Ibias almost exactly because the DC gain of the integrator, coupled with the additional gain produced by the transimpedance stage,
is very high. Consequently,the integrating control loop completely overrides the current feedback loop at DC and the output offset is reduced from that given by Equation(4) to:
Voos=Vios(A2)X(1+R11/R10)
This means that it can be made arbitrarily small through the choice of a low offset amplifier for A2.
Here the cost of an additional op amp is more than offset by not having to use expensive matched NPN and PNP dual transistors in the current mirrors.
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楼主
发表于 2015-6-3 21:29
