Best way of measuring output impedance of a VAS?

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godzich
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Best way of measuring output impedance of a VAS?

Post by godzich »

Hi,

Looking for a standardized or easily repeatable way of determining the output impedance of a VAS stage - and/or - determining the output impedance of a current source or sink. To do this in the simulation is fairly easy (good ideas welcome here too) - but how to do it in a real circuit without interfering too much with the actual circuit? It seems difficult to measure reliably in circuits with a fairly high output impedance.

Any good ideas och schemes?

Cheers,

Christian
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JR.
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Re: Best way of measuring output impedance of a VAS?

Post by JR. »

I had to google VAS and it appears to mean "Voltage amplification stage".
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Are you talking about the intermediate voltage gain stage between a differential front end and the output stage buffer (unity follower).

By definition current sources/sinks have high output impedance and rely upon negative feedback to manage output voltage.

I am not sure i understand the question. When designing a multistage amplifier the important criteria is generally the ability to supply adequate drive current for the output stage.

Perhaps you can be more specific about the topology you are working on. To measure source impedance perhaps drive a cap coupled resistor open loop, but you will need some DC feedback to bias up the circuit and this is not a pure source impedance measure but output current vs input voltage (transconductance?). I have never tried this.

JR
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godzich
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Joined: Tue Nov 08, 2016 10:10 am

Re: Best way of measuring output impedance of a VAS?

Post by godzich »

Hi,

Lets simplify the case and rephrase it as;

how to properly measure the output impedance of a current mirror? This can be done in computer simulations quite easily by loading the current mirror output with a ridiculous size inductor to pass the mirrors DC-current not loading the AC, and then injecting a known AC current to the output (or voltage via a known resistance) and calculate the impedance from there.

But how do you do it practically in a real circuit? I don't have on hand any reasonable sized 10k Henry inductors ;) Whatever you put there to measure the output impedance (that can be of Megaohms magnitude) should not interfere with the measurement. Especially important when measuring the output impedance at higher frequencies... any additional capacitive loading would interfere with the measurement...

Christian
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JR.
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Re: Best way of measuring output impedance of a VAS?

Post by JR. »

godzich wrote:Hi,

Lets simplify the case and rephrase it as;

how to properly measure the output impedance of a current mirror? This can be done in computer simulations quite easily by loading the current mirror output with a ridiculous size inductor to pass the mirrors DC-current not loading the AC, and then injecting a known AC current to the output (or voltage via a known resistance) and calculate the impedance from there.

But how do you do it practically in a real circuit? I don't have on hand any reasonable sized 10k Henry inductors ;) Whatever you put there to measure the output impedance (that can be of Megaohms magnitude) should not interfere with the measurement. Especially important when measuring the output impedance at higher frequencies... any additional capacitive loading would interfere with the measurement...

Christian
It is still unclear why you need to do this.

The output of a current source by definition is very high, so will be difficult to measure.

Any reason for not using a resistor? I will have to speculate about the circuit but imagine a large value resistor in series with a cap connected to a sine wave source. By feeding a 1V sine wave through say a cap and 1M resistor, then looking at the attenuation or padding caused by the current source you can impute the impedance of the current source output. For example a 6dB loss would suggest a 1M source impedance.

The only time I measured anything remotely like this was back in the 80's when I designed a current source sum bus, and to measure that I injected a voltage into the + input of a virtual earth sum amp. For that case the source impedance of my current sources became part of the gain equation (1+Rfeedback/Rsource impedance).

You could do a similar test with an op amp. Connect a 1V AC signal to the + input of an op amp. Use a nominal feedback resistor from op amp output to - input. Then connect the - input to your current source node you want to measure. Your test op amp output will be increased by the effective R source impedance from the current source.

JR
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