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Re: why opamp is called voltage controlled voltage amplifier
Depending on the fuction that the amplifiers may perform, it is design for diferent combinations of inputs and outputs signal, so you can find
V-V
V-C
C-V
C-C
V=voltaje
C=courrent
Re: why opamp is called voltage controlled voltage amplifier
There are two main categories of Operational Amplifiers:
1: a Generic op-amp is a "Voltage Controlled Voltage Source"
2: a Norton (transconductance) op-amp is a "Voltage Controlled Current Source"
All op-amps are "differential amplifiers" (have plus and minus inputs)
* a positive voltage change at the plus input causes a positive output shift
* a positive voltage change at the minus input causes a negative output shift
* the "output" of an Generic op-amp is a controlled voltage
* the "output of a Norton op-amp is a controlled current
The basic "open loop" mathematical models are:
* Generic: Vout = G x (Vplus - Vminus), where "G" is the open loop gain
* Norton: Iout = gm x (Vplus - Vminus), where "gm" is the transconductance
Both G and gm are significant so that the output will "saturate" with little change at the input (ie. an open loop operational amplifier behaves like a digital comparator).
A stable linear op-amp circuit requires a "negative feedback loop" around the device. The op-amp output is connected to the minus input via a circuit designed to force the minus input voltage to equal the plus input voltage.
The "input signal" is connected to either of the op-amp inputs. If the minus input is used, it must be connected by a circuit to the feedback loop, and the output will be an inverted function of the input signal.
A negative feedback loop stabilises an op-amp circuit by limiting (negating) its effective gain. The lower the effective gain, the more quickly the op-amp can reach its target output, and the faster input signal changes it can follow. This is referred to as the gain/bandwidth product.
If the op-amp output is only connected via a feedback network to the plus input the result will be an oscillator.
A generic op-amp is designed to have very high input impedances, so for basic applications you can assume no current flows into the op-amp inputs. This simplifies the design and analysis of feedback networks.
Beginners always learn how to design generic op-amp circuits first. So try looking at the difference between an inverting and non-inverting amplifier circuit and remember the points from above.
P.S. A Norton (a.k.a transconductance, a.k.a current feedback) op-amp can operate at higher frequencies than a generic op-amp. But the minus input, unlike the plus input has a very low impedance, and can be approximated as a diode connected to the negative op-amp supply rail, so current will flow into it. Thus the input signal is always connected to the high impedance plus input. This also makes feeback networks a little more complicated. If the resistance of a feedback network from the output to the minus input is too low, the feedback current will be too high and it will destroy the input.
Re: why opamp is called voltage controlled voltage amplifier
It's really just a simple case of what property controls what property.
Op Amps are voltage controlled voltage amplifiers. A = Vout/Vin(diff)
Transistors are current controlled current amplifiers. beta = Ic/Ib
FETs are voltage controlled current amplifiers. gm = Ids/Vgs
I think there are transresistance amps too, ie current controlled voltage amps, but I'm not too familiar with them.
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