# why are two op-amps required in some circuits if one op-amp can provide high gain?

#### testuserabcdef

Joined Jul 12, 2016
127
I'm trying to make my own rfid reader based on this:

But what I want to know is why are two op-amps and a transistor necessary? Couldn't one get away with one op-amp and a higher negative feedback resistor value and still achieve the same gain? or is there something I'm missing?

#### davideather

Joined Dec 12, 2016
33
Maybe... but he probably already tried. As gain goes up the bandwidth of the OP amp goes down (proportionally) and he needs at least 125kHz

#### ronv

Joined Nov 12, 2008
3,770
I'm trying to make my own rfid reader based on this:

But what I want to know is why are two op-amps and a transistor necessary? Couldn't one get away with one op-amp and a higher negative feedback resistor value and still achieve the same gain? or is there something I'm missing?
The transistor makes sense because it squares up the signal from the first op amp. The second one doesn't work at 125 Khz because it saturates, so won't switch that fast.

#### Papabravo

Joined Feb 24, 2006
14,871
In circuit design: gain is not the only consideration. There can be a list of reasons for using two amplifiers as long as your arm.

#### crutschow

Joined Mar 14, 2008
26,053
But what I want to know is why are two op-amps and a transistor necessary?
Generally it's not just the circuit gain that's important, there's also the circuit bandwidth.
Each op amp has a bandwidth equal to its gain-bandwidth divided by the closed-loop gain.
Thus two opamp in series with the same total gain as one op amp will have twice the bandwidth.

#### atferrari

Joined Jan 6, 2004
4,172
Thus two opamp in series with the same total gain as one op amp will have twice the bandwidth.
Hola Carl

First time I see that. Could you please elaborate?

#### BR-549

Joined Sep 22, 2013
4,938
It's because of different transducer outputs and the required signal inputs for processing or transmission.

Some transducers output a current and some output a voltage. And because of signal characteristics....we might need to convert the current to voltage....or convert the voltage to current.

One might need to select (tune) a particular parameter. And as mentioned, one might need to filter more than one parameter(band) also.

#### bertus

Joined Apr 5, 2008
21,054
Hello,

Thus two opamp in series with the same total gain as one op amp will have twice the bandwidth.
I would not agree with this.
Take this example:
1 opamp GBW 1 Mhz gain 100 will result in an effective bandwith of 1 Mhz / 100 = 10 Khz.
2 opams with each GBW 1 Mhz gain 10 will result in an effective bandwidth of 1 Mhz / 10 = 100 kHz.
As there are 2 opamps used the total gain is 10 X 10 = 100.

Bertus