How to measure the 3 dB bandwidth of an opamp in LTSpice

Question

I am interested to use an opamp with an exact 3 dB rolloff at 320 kHz for an sinewave (voltage swing) from -10 to +10 V.

I think I need to do this using some resistors and capacitors on the feedback path.

I tried to simulate this using LTspice to find the values, but my simulation does not seems to be correctly set up.

What is missing from the simulation circuit or the directive?

There is no DC feedback path from the output to the (-) input so the DC gain equals the open-loop gain of the op amp, thus it is saturated and does not amplify the AC signal. Do a "DC op pnt" test to look at the bias voltages and you will see this. — Carl Rutschow, Commented Sep 3 at 14:01
@CarlRutschow Do you mean the feedback of the should be behind the R3 resistor? — DEKKER, Commented Sep 3 at 15:41
@CarlRutschow Would you please give a hint as what "no DC feedback path" means? — DEKKER, Commented Sep 4 at 6:42

Arsenal · Accepted Answer · 2024-09-03 14:11:58Z

4

The issue you are having is that prefixes are not case sensitive in LT Spice.

So M = m = milli and not mega like expected by many new users.

To have mega you need to use meg.

So change your simulation rule to 3meg instead of 3m and your simulation will run to 3 MHz.

I won't comment on the circuit design, been away too long to give a good answer for that.

answered Sep 3 at 14:11

Arsenal

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\$\begingroup\$ I usually use E notation for things like this, so my simulation command would looks something like .ac oct 200 1 3E6 where 6 is the multiplier, 3 for kilo, 6 for mega, 9 for giga, etc. \$\endgroup\$
– GodJihyo
Commented Sep 3 at 15:12
1

\$\begingroup\$ @GodJihyo I rarely use E notation, so everyone is different and SPICE users should be able to decode both. I agree though that if using E notation, using it in multiples of 3 is much preferred. \$\endgroup\$
– Ste Kulov
Commented Sep 3 at 18:31

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