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Transfer function again.

I'm not sure I can explain it better but I'll try. The first thing you must realise is that the phonocube does not amplify a voltage, it converts a current to a voltage. It is acting as a transimpedance amplifier, the operation of which is pretty standard in instrumentation. As I will explain, there is virtually no voltage present at the input so there is no point talking about voltage gain.

There is however a current present at the input, and since moving coil cartridges operate happily as current sources this works very well. Without boring you with op-amp feedback theory (yet), suffice it to say that output will rise to a level where the current in the feedback loop balances the input current, and the amplifier therefore has a definite ratio of output voltage to input current. Since it is transferring one quantity (current) at the input to another quantity (voltage) at the output this ratio is properly called a transfer function. I calculate that the transfer fuction of the 47 labs unit is around 10 volts per milliamp on the low gain setting.

We can find the closed circuit output current of any moving coil cartridge to a reasonable approximation by considering it as a current source shunted by its own coil resistance. The value of the current source is thus its open circuit output voltage (the output voltage normally specified) divided by the shunt value. This has nothing to do with voltage ratios at the input or any of the guff written by the reviewers, it is simply derived from Kirchoff's Laws.

So we have a specified current and a definite transfer function. If we multiply one by the other, we get a definite output voltage. Simple.

If you want to know how a transimpedance amp works, read on.

The input is directly to the inverting input of an op-amp, with the feedback loop also connected from the output to the inverting input. The non-inverting input is grounded. The function of the loop is to keep the voltage at the inverting input equal to the voltage at the non-inverting input. As I said the non-inverting input is grounded so its voltage is zero and the inverting input is thus also kept as close to zero voltage as the op amp can achieve. This means that whatever current you put in to the input, almost no voltage appears at the input, so it has zero input impedance. The voltage at the input is actually the output voltage divided by the open loop gain of the op amp so the effective input voltage is a function of the amplifier performance. Talking of voltage gain is therefore meaningless.



Mark Kelly


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  • Transfer function again. - Mark Kelly 21:46:16 12/02/05 (2)


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