OpAmp Circuit Bandwidth calculation

[hgg]

Hi,

Just watched Dave's video on opamp cascading in order to increase bandwidth which
is a nice trick indeed, but it got me thinking about my own little circuit that I am using
with the pulsing light amplifier I am working on.  While I can measure low lux light
pulses of 60khz I think that the actual bandwidth of my circuit is really low...



Can anybody help me measure the bandwidth from the diagram below?
I am using the TI LM358P dual opamp with a unity gain bandwidth of 0.7Mhz
http://www.ti.com/lit/ds/slos068r/slos068r.pdf

Maybe I can use this trick and a better selection of components to increase
the bandwidth.

Thanks.


OpAmp Circuit

[Rerouter]

your diode has a maximum bandwidth of about 5MHz, as for the circuit your dealing with AC gain which is dependent on frequency

[hgg]

Do you mean that you can calculate the bandwidth only if you know the frequency
of the AC input signal?

What would then be the bandwidth of the circuit in the case of a 300khz input for example?

[Andy Watson]

If speed is your desire, consider reverse biasing the diode so that it operates in photo-current mode.

[hgg]

I am aware of that, but what I want to know is how to calculate the max bandwidth of my circuit.

[Rerouter]

with the model made in spice (you will need to find the model for your exact op amp, but it should be available) and adding the parasitic ESR of C3, you should be able to perform a bode plot, this is a function inside of spice,

[hgg]

So is it that difficult to calculate it by hand?  Only with simulations?

What about the gain?  Can you calculate the gain?

[Rerouter]

ok it can be done by hand but it only tells you what the gain should be as opposed to if the op amp is capable of doing it,

C2 at 300Khz would have an impedance of 5305 ohms assuming almost no ESR
C3 at 300Khz would have 5.305 ohms,

And i have no idea how to even begin on the source impendance of a pin diode, so lets just assume its a nice low impendance voltage source,



so R1 in this case would be 5.3 ohms,
R2 would be 5300 ohms (4 mega ohm resistance contributes almost nothing at this frequency)

so your gain is set at 1000 at 300Khz, which is well past what your op amp is capable of at this frequency,

The bode plot is helpful here as you can play with values to help reduce your gain to more sane figures, obviously if the diode has a high source Independence the gain dramatically drops, and lowering the value of its AC coupling cap would reduce the gain aswell,

now if we where to take 120hz for an incandescant bulb your gain is ,

R1 = 13262 ohms
R2 = 13262911 ohms in parrellel with 4 Meg... so =  3.07 Meg

so your gain is 232 even at this low frequency...

[hgg]

Rerouter, thank you for your explanation!
You reminded me some points that I forgot to take into account,
like the behaviour of a capacitor and its impedance at high frequencies...

[free_electron]

If speed is your desire, consider reverse biasing the diode so that it operates in photo-current mode.

bingo.

this contraption with a capacitively coupled diode … meh ...