# Op Amp gyrator oscillates

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#### whoandcar

I have built an op amp impedance gyrator to apmplify the signal from an IR photo diode at around 5 KHz.
The circuit is very simple, but it self-oscilates if the supply voltage is lower than 9.5 volt. At higher Vcc it performs very well. But occasionally Vcc drops below 9.5 and the oscillation is there again.
Any ideas?
The op amp is a TL062 as I need it to be low power. It is close to the upper frequency limit but goes OK.

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#### Andy L. Francis

Oscillates in what frequency?

#### SY

This sort of topology is a one terminal device- you don't use the terminal that you're marked "output" It just acts as an inductor from the input to ground. If you use this as the bottom leg of a frequency-dependent voltage divider, do you see oscillation at the input?

#### whoandcar

Wow! I didn't yet finish my coffee and I got 2 answers! Thank you.

It oscillates at a frequency that is variable according to supply. Below 9.5 Volts it is aroud 2 KHz and lowers with less voltage.

The input isn't unterminated, it is connected directly to a BPW41 photo diode (in reverse: anode to Gnd, cathode to input)

The idea is to present low impedance to DC (2 K Ohms) to help with de DC current that is present in even undirect sun-light, but high impedance at the working frequency of 5 KHz (near 1 M Ohm).

#### jcx

if the circuit manages to look like an inductor then LC resonance could be the issue with a little extra phase shift from the op amp nudging the synthetic "L" over to show some added neagtive resistance

the C of the photo diode would vary inversely with bias V

its more common to use a transconductance circuit, but these can have stability issues too due to the photo diode C

you could try a bootstrap current source:

series split bias R to the supply, unity gain buffer the diode V, connect a bootstrap C from the op amp out to the midpoint of the bias R string

#### whoandcar

That is why the oscillation frequency lowers with lower Vcc? (more capacitance)
You are a genious!
I need a low DC resistance is series, 'cause my photo diode is mounted in a car and the ambient light can vary from almost direct sunlight to complete darkness.
I'll work on the diode capacitance and will keep you informed.
Thank you for the idea.

#### jackinnj

Burr-Brown had an entire section of their book on Linear Applications devoted to photo-diodes. Look for the following application notes on Texas Instruments website:

"Photodiode monitoring with op amps"
"Designing photodiode amplifier circuits with opa128"
"Tame photodiodes with op amp bootstrap"

#### whoandcar

Solved!

SY said:
This sort of topology is a one terminal device- you don't use the terminal that you're marked "output"

I apologize 'cause I didn't read your post very well the first time.

Off course it is a "one terminal" device. I use the output of the operational due to its lower impedance to couple the next stage.
The "output" is an exact AC replica of the photodiode voltage.

As usual, the problem lied outside the shown circuit! The decoupling capacitor of the bias for the + input of the op amp was too low. Rising the value from 0.1 uF to 1 uF solved all my troubles.

But you made me think

First I disconnected de photodiode. Oscillates. Disconected the 6 inches of shielded cable. Oscillates. Disconnected all the following stages. Oscillation. There was nothing else to disconnect! The bare gyrator oscillated by itself. My supply is very weak, and it seems to be a small feedback to the input. In-phase.

The only thing that remains unclear is why it stopped oscillating and performed OK above 9,5 Volt? (All the way up to 15 Volt) Today I tested it with as low as 5 Volt and it performed nicely!

I also measured the inner capacitance of the photodiode. Just for the curious:

40 pF @ 12 V
80 pF @ 9 V
120 pF @ 6 V
240 pF @ 4 V

It rises insanely below 6 Volt!

Thank you all!

#### unclejed613

decoupling the power supply rails is VERY important. you have internal capacitances in the transistors in the op amp. and just like the photodiode, those capacitances are inversely proportional to the applied voltage. on the output stage of the opamp, one or more of those capacitances (Cbc) are connected directly to the supply rails. these capacitances will couple some of the signal into the power supply.

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