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suvendu
Joined: 10 Oct 2004
Posts: 442
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 17 Jun 2005 10:44 why opamp is called voltage controlled voltage amplifier? |
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| why opamp is called voltage controlled voltage amplifier?give strong reason.
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flatulent
Joined: 19 Jul 2002
Posts: 4875
Helped: 324
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17 Jun 2005 17:09 Re: why opamp is called voltage controlled voltage amplifier |
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| The output is a voltage. The input is a voltage. There are also courrent input controlled amplifiers called OTA.
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Neo_82
Joined: 29 Apr 2004
Posts: 37
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17 Jun 2005 18:00 Re: why opamp is called voltage controlled voltage amplifier |
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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
depending of that the literature gave them a name
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smyback
Joined: 15 Feb 2004
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20 Jun 2005 20:38 Re: why opamp is called voltage controlled voltage amplifier |
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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.
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20 Jun 2005 20:38 Ads |
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checkmate
Joined: 25 Feb 2004
Posts: 489
Helped: 35
Location: Toilet Seat
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21 Jun 2005 10:08 Re: why opamp is called voltage controlled voltage amplifier |
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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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