# Some questions regarding Op-AMPs

#### circuits12

Joined Dec 24, 2011
2
Hello there, just seen that great board and thought you guys may help me to figure out those questions about operational amplifiers.

1-What is the bandwidth on op-amps exactly, and its relation with gain ?

2-What is the function of the feedback resistor ?

3-Why does the gain diminish as frequency increases to high values ?

Joined Dec 26, 2010
2,148
This looks like a series of assignment questions, perhaps an on-line quiz? We don't provide direct answers to that kind of stuff here.

Gain, bandwidth, and feedback are basic subjects, covered on the ebook pages on this site, but also very suitable for looking up using a search engine.

#### circuits12

Joined Dec 24, 2011
2
No, indeed those are the concepts I'm trying to understand to use them on a large scope, but thanks though

#### justtrying

Joined Mar 9, 2011
432

Joined Dec 26, 2010
2,148
Hello there, just seen that great board and thought you guys may help me to figure out those questions about operational amplifiers.

1-What is the bandwidth on op-amps exactly, and its relation with gain ?

2-What is the function of the feedback resistor ?

3-Why does the gain diminish as frequency increases to high values ?

• The bandwidth of any amplifier can mean one of several things:

1. The frequency range over which a certain small-signal gain is obtained: for op-amps this is usually from DC to some maximum frequency. Often the unity-gain bandwidth is quoted, that is the upper frequency at which the gain falls to one.
2. For many op-amps, the gain for which the amplifier is configured multiplied by the bandwidth in that configuration is approximately constant for a wide range of different gains.
3. The frequency range at which the gain has dropped by a stated amount (often 3dB) from its mid-band level. This is more usually stated for a completed amplifier circuit than for an op-amp as a component.
4. The large-signal or power bandwidth is the maximum frequency at which full output level (or a specified fraction of it) can be obtained. Typically this is set by slew-rate limitation.

• Many feedback configurations are possible, so it is not simple to say what is the function of "the feedback resistor". The most common op-amp configurations have a resistor connected to the output to obtain voltage feedback.

1. This can be connected to the inverting input to obtain negative feedback, or to the non-inverting input for positive feedback.
2. The feedback will be classified as shunt if it is connected to the same terminal as the input: if not, it is series feedback.

• The gain of any amplifier will diminish at a sufficiently high frequency, due to parasitic capacitances, inductances, and such things as transistor speed limitations. A deliberate falling response is often incorporated into operational amplifiers however, because they will often be used with heavy negative feedback. This is described as frequency compensation. The aim is to ensure that the amplifier gain drops to unity before phase delays around the feedback loop can become too large, as otherwise the circuit may oscillate.