Does Op-amp/IA work as a voltage amplifier or current amplifier

Thread Starter

Xin_99

Joined Nov 18, 2022
4
Hi, maybe i know doest the op-amp work as a voltage amplfier or current amplifier or both?
Is't possible to work as a current amplifier by just using an "instrumentation amplifier"?
 

crutschow

Joined Mar 14, 2008
31,151
Both an op amp and an instrumentation amp are voltage amplifiers.
You can convert an op amp into a current-amplifier with some additional components.
Look up Howland current amplifier for example.
 

MrChips

Joined Oct 2, 2009
27,727
Depends on what you mean by current amplifier.

Amplifiers in general are voltage amplifiers. They take an input voltage and output an output voltage given by
Vout = Voltage gain x Vin

Every voltage amplifier can become a current amplifier.
Vin = Iin x Rin
Vout = Iout x Rout

Substituting into the first equation,
Iout x Rout = Voltage gain x Iin x Rin
Iout = Voltage gain x Iin x Rin / Rout
 

MisterBill2

Joined Jan 23, 2018
13,845
Many amplifiers that amplify the input voltage to a greater output voltage will also increase the output current relative to what that current would be if the load were connected to the input voltage source.
But that is not what is meant when using the term "current amplifier." The previous explanations are correct.
 

DavidInOz

Joined Nov 8, 2022
8
I have been designing analog circuit for 60+ years. The difficulty with the idea of a current amplifier c.f. a voltage amp is this: Voltages are more often than not measured relative to Gnd or circuit common. Currents very seldom are. Currents flow through wires, and each case is different in terms of what voltage that wire is at. And real circuits are powered by voltages, say +5V or +/15V or whatever, almost always referred to some real or hypothetical Gnd.

So, while current amplifiers are of course possible, each design needs to be made in the context of what that current is and represents.

For example, at one time a very common industrial signalling method was 4-20mA current loop. A constant current drive is less likely to be upset by ambient hum, earth potential differences, etc. But transmitters and receivers for 4-20mA are sometimes quite complex and difficult to design, because at the end of the day the parameter being measured and transmitted will generally be - you guessed it - a voltage. Also, in industrial applications you need galvanic isolation in at least one place in the circuit - there can be tens of volts of earth potential difference between spots hundred of metres apart. I have the lumps to prove that!

As for an opamp working as a current amplifier, yes it can be done but only in a very limited context. You can even do it with a few transistors, again in a limited context, Look up current mirror.
 

WBahn

Joined Mar 31, 2012
27,942
Operational Transconductance Amplifiers are capable of current gain in that the output current is proportional to a control current.
Check Table 1 in this document:

https://www.ti.com/lit/an/sboa117a/sboa117a.pdf
If it's a transconductance amplifier, then the output current is proportional to a control voltage -- they are voltage-controlled current sources.

Now, with some added components, it's certainly possible to obtain the control voltage from a control current (a suitable resistor is likely all that is needed).
 

WBahn

Joined Mar 31, 2012
27,942
Hi, maybe i know doest the op-amp work as a voltage amplfier or current amplifier or both?
Is't possible to work as a current amplifier by just using an "instrumentation amplifier"?
The basic opamp is a voltage amplifier with extremely high gain. They are pretty much useless by themselves and only become useful when incorporated into a larger circuit that includes external components that, usually through negative feedback, causes the overall circuit to behave in a desired fashion. That overall circuit could be any of a lot of different things, among them a voltage amplifier or a current amplifier or neither of the above.
 

Yffig

Joined Aug 25, 2019
18
There are not that much differences: check the Analog Devices Datasheets for Hi-Speed AD8001 (Current Feedback) and ADA4817 (Voltage Feedback). (Spice models are easily found on the Internet for integration into LT-Spice). Both are used the same way for low gains (due to hiSpeed) amps. Read the "Theory of Operation" section for AD8001.
 

vanderghast

Joined Jun 14, 2018
64
If you can convert the voltage to a current through a highly precise resistor, you must also check the maximum output CURRENT the OpAmp that you are using can deliver. It can be quite low, in the mA range. There are some OpAmp that can output a current in the 100A, but they are not given (closer to the 1k$ mark, each, than to 1$).
 

MisterBill2

Joined Jan 23, 2018
13,845
Interestingly enough, converting that 4 to 20 mA signal back to a voltage signal works well with an instrument amp circuit and a 50.0 ohm resistor. We did that quite a bit. We still had to be careful to stay within the common-mode voltage rating of the IA, but as we powered those with +/- 15 volts that was not super difficult.
(sorry about being a bit off topic).
 
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