Transimpedance amplifier

Discussion in 'Electronic Design' started by [email protected], Jan 11, 2009.

1. Guest

Hi,
i've simulated a transimpedance amplifier with spice. My circuit is
composed of an opamp and a feedback resistor; in parallel to current
source a capacitor is present (i want to use a photodiode's equivalent
model). I have a question: is it possible that (with an ac sweep), V/I
transfer function is greater (at lower frequencies) than opamp open
loop gain? Why?
Another question: i wanto to use it as input of a sar adc. I've read
this interesting article
www.edn.com/article/CA6602451.html
and i want to try it. Is it more correct to use another opamp as
buffer and rc circuit as its load (as describend in the link) or use
rc load directly to transimpedance amplifier?
Thanks

2. MooseFETGuest

The open loop gain spec you are looking at may be a minimum and the
model may use the typical.

You didn't say there was a peak but if you see a peak in the curve
just before it falls off the answer may be the phase margin of the
system.

Beware of EDN articles they are too often bogus. Does your ADC have
sample and hold in it? What sort of load does it present to its
input?

You may or may not need another op-amp in the system at all.

3. Guest

How can i verify it?No opamp's macromodel is present, because my opamp
is realized with transistor...
Yes, a peak is present....i've read about on graeme's book.I've used a
feedback capacitor to remove it
It has a sample and hold with r=on switch resistor=100ohm and
c=sampling capacitor=5pF
thanks

4. MooseFETGuest

Making the peak go partly away but not completely can lead to a better
system. A slight bump before the gain starts to fall can move the 3db
point up.
The output impedance of the driving circuit needs to be far less than
the 100 Ohms. The output will actually look inductive. Since you are
modeling the system, you can model the sample and hold action too.

5. Guest

x-no-archive:
Well the "open loop gain" is measured in Volts out / Volts in and the
V/I transfer function is measured in Volt out / Amps in..... so you
have an apples and oranges problem I would think.

Mark

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