mediatechnology wrote: ↑Tue Feb 19, 2019 7:03 pm
I haven't experimented using this servo with switched gains but I think it has advantages.
Olaf alerted me to this technique in a mic preamplifier thread.
Since I had a similar topology on my bench in the form of the MC preamp I decided to try it there.
It worked so well I decided to use it even though the MC preamp is essentially fixed-gain.
It's something I definitely intend to try in a variable gain mic preamp.
Great.
I studied injecting the servo correction at the inverting inputs of the op-amp versus the non inverting inputs.
I came to the conclusion it makes no significant difference. I convinced myself this design choice does not make a difference about fc shifting with the gain.
Using all symetric circuits, no additional 11V rail, just using the +15V -15V rails, here is what I come up with.
In both cases I have a +1mA / -1mA at the transistors collector current from 3V servo output. Gain here is a transconductance 0.3333mA/V
In both cases, Ic is 6mA, 5mA, 4mA for servo drive 3V, 0V, -3V.
Case A: Injection at the non inverting inputs is an easier load on the servo that must drive like 15K to ground.
Collector resistor is 1K to the 15V rail.
Case B: Direct injection at the transistor collectors puts a heavier load on the servo that must drive like 1.8K to ground.
Collector resistor is 0.6K to the 15V rail.
This +1mA -1mA collector current -assuming infinite high beta transistors- will result in the same +1mA -1mA current throught the Rfeedback resistors. So with Rf =1K it gives +2V at the op-amp outputs.
Finally: When the outputs of the Deboo send +3V -3V to skew the mic preamp,
it results in +1V -1V at the op amp outputs,
which results in +2V -2V at the THAT1240 outputs
The good news is that this does not depends of the chosen preamp gain. Indeed fc doesn't shift with changing the mic preamp gain.
BUT there is a catch: This is only true if the transistors have extremely high beta. The bad news is that this is wrong in the case of not so high beta together with high values of transistor base resistors ( I plan on using two 10K where you have two 0.499K ) and this becomes terrible at high preamp gains like 60dB. The trouble comes from the voltage drop at the base resistors.
I am working on this, with some compensation scheme, not easy because Beta is not well defined and has tempco.
Well, in order to have fc not shifting with gain, I must solve this trouble or compromise with not so ambitious expectations.