What is transimpedance?

Question

I've googled almost 100 times, unable to find what transimpedance really is. Every search displayed results about transimpedance amplifiers, but didn't explain the term transimpedance.

Now when you Google it, you'll come here! – JYelton Jul 24 '13 at 00:59 — JYelton, Jul 24 '13 at 00:59

score 24 · Accepted Answer · answered Jul 23 '13 at 18:19

Impedance means a circuit element that produces a voltage when a current is applied.

For example, if you apply 1 A to a 1 Ohm resistor, 1 V will be generated across the resistor.

Transimpedance applies to a 2-port (or n-port) device, rather than a simple single-branch circuit element, and it means the two-port produces a voltage when a current is applied, but the voltage appears on a different port than the current was applied to.

Describing a device with a transimpedance means you're describing the device as a CCVS (current-controlled voltage source). Like an ordinary impedance, the units of a transimpedance are ohms (V/A).

As an example, a transimpedance amplifier produces a voltage at its output when a current is provided to its input.

score 11 · Answer 2 · answered Jul 23 '13 at 22:07

Consider a amplifier that takes current as input and produces a voltage proportional to that current. How would you define its gain?

In a normal voltage-in voltage-out amplifier, the gain is the change in output voltage divided by the change in the input voltage that caused it. This is a dimensionless quantity since the units cancel out when you divide a voltage by another voltage. For example, you could say such a amplifier has a "gain of 10".

For a current-in voltage-out amplifier, the gain is the change in the output voltage divided by the change in the input current that caused it. This time we are dividing a voltage by a current, which has units of resistance. It wouldn't make sense to say such a amplifier had a "gain of 10" because that is dependent on the units used to measure the output EMF and the input current. For example, are you talking about Volts per Amp, Volts per milliamp, or something else? However, you could say the gain is 10 Volts per Amp, which is the same as saying the gain is 10 Ohms. That means the output voltage is the same as a 10 Ω resistor would generate if the input current were passed thru it.

This property of current-in and voltage-out is called transimpedance. A amplifier specifically meant to do that is called a transimpedance amplifier. The term "transimpedance" can also be used for the gain of such amplifier. For example, you could say "This amplifier has a transimpedance of 1 kΩ.".

Alfred Centauri · Answer 3 · 2013-07-24T03:08:10.033

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Transimpedance is short for transfer impedance. It is the ratio of the voltage across the output port to the current into the input port. In other words, it is the gain of a current to voltage converter.

This is to be contrasted with driving point impedance which is the ratio of the voltage across, and the current into, the same port.

The dual to this is transadmittance or transfer admittance, the ratio of the current out of the output port to the voltage across the input port.

edited Jul 24 '13 at 03:08

answered Jul 23 '13 at 22:12

Alfred Centauri

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Isn't transadmittance more commonly referred to as transconductance? The references I've found handy online seem to tend to place transadmittance in the context of AC signals and vacuum tubes. I'm not sure if they are synonyms and one term is just in more current usage or if there is an actual distinction. They both appear to represent Iout/Vin. – scanny Jul 29 '15 at 04:45

score 3 · Answer 4 · answered Jul 23 '13 at 21:49

A 1 ohm resistance produces 1V across its terminals for every 1 amp flowing between its terminals. It's resistance is 1 ohm and it's impedance is 1 ohm.

The "trans" part of transimpedance is like trans-continental - it means "across" or "between" or "covering an area of" and is added to the word "impedance" to suggest that there is an input and an output with something that "covers" the ground in between.

If a resistor were encased like a chip with two terminals one one side (i.e. an input) and, two terminals on the other side (like a video line driver) and someone said it was a 10k transimpedance amplifier you'd feed 1mA in on the input side and expect to see 10V on the output side.

The real difference between this 10k resistor being a transimpedance amp and an actual op-amp being inside is that the input to the transimpedance op-amp would look like a short circuit but it would still produce 10V on it's output for 1mA in.

Does this make sense?

What is transimpedance?

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