20-02-2018, 08:21 AM
I can't see that increasing Z out will make much difference to frequency response. In any case the Z out at higher frequencies will differ from that at low. If only due to stray C. But we're only talking about audio here where the performance of the CCS is going to be pretty good.
BTW, you can't have low Z at DC and high Z for AC using a CCS. The definition of an ideal CCS is that it has infinite Z at all frequencies. What that means at DC is that if you change the applied voltage slightly then the current hardly varies.
Compliance is the range of voltages over which a CCS will work. For your sort of design the upper limit is set by blowing up the transistor, the lower limit is set by taking the collector voltage below the base voltage.
Trying to measure Z out by paralleling with a resistor is bound to fail for a CCS like this. Conceptually you can do it by applying a variable voltage to the output of the CCS and measuring the current. In a working circuit you can 'scope the emitter as the collector current is close to the emittter current. This will also give you some idea of the dynamic performance of the CCS.
BTW, you can't have low Z at DC and high Z for AC using a CCS. The definition of an ideal CCS is that it has infinite Z at all frequencies. What that means at DC is that if you change the applied voltage slightly then the current hardly varies.
Compliance is the range of voltages over which a CCS will work. For your sort of design the upper limit is set by blowing up the transistor, the lower limit is set by taking the collector voltage below the base voltage.
Trying to measure Z out by paralleling with a resistor is bound to fail for a CCS like this. Conceptually you can do it by applying a variable voltage to the output of the CCS and measuring the current. In a working circuit you can 'scope the emitter as the collector current is close to the emittter current. This will also give you some idea of the dynamic performance of the CCS.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
