# Photodiode Amplifier Design

#### Sergen Erbay

Joined Dec 19, 2019
20

What is the purpose of the current section?

#### ericgibbs

Joined Jan 29, 2010
9,536
Hi,
It sets the steady state Vout at Vcc/2
If R3 = R2
E

#### Sergen Erbay

Joined Dec 19, 2019
20
When the photodiode does not generate current, will the circuit output be produced from here?

#### ericgibbs

Joined Jan 29, 2010
9,536
hi,
Consider that you are using a single supply for the OPA.
Say you wanted the Vout to swing positive and negative when there is a signal from the photo-diode.
With a single Vcc , Vout cannot swing below zero volts, so a common way to overcome this limitation is to bias the Non Inv input of the OPA to a positive voltage level.
This raises the steady state of Vout above zero volts, usually R2=R3 so that Vout is Vcc/2.
Update:
So the photo diode will drive Vout above and below Vcc/2
Is this what you are asking.?
E

#### Wolframore

Joined Jan 21, 2019
1,609
it’s called a virtual ground to set the zero point of an opamp working in DC.

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#### Audioguru again

Joined Oct 21, 2019
858
The inputs of most opamps do not work if their voltage is within a few volts from the negative supply voltage. Since your circuit has no negative supply voltage, instead it has 0V then the opamp inputs must be biased to a positive voltage that is away from your 0V.
Also, the photodiode needs a reverse bias voltage across it.

#### Sergen Erbay

Joined Dec 19, 2019
20
I had a new question, what is the purpose of the red area in the circuit?

#### ericgibbs

Joined Jan 29, 2010
9,536
hi,
It is wired as a Comparator, with hysteresis, R8 and R9 set the threshold voltage level.

E

#### Wolframore

Joined Jan 21, 2019
1,609
I agree it’s a comparator and it would have hysteresis if we we using -/+ rails. I believe that biased with DC it only triggers at half point since the capacitor coupling centers the input.

#### ericgibbs

Joined Jan 29, 2010
9,536
hi W,
R12 is connected from the Output back to the NI input of the OPA, so as NI is driven high by the incoming signal via C4, the Vout going adds to the to NI input, ie; hysteresis.
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#### Wolframore

Joined Jan 21, 2019
1,609
you're right of course!

#### ericgibbs

Joined Jan 29, 2010
9,536
hi W,
So that the TS can see his output stage, here it is in LTSpice,
Eric

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#### Wolframore

Joined Jan 21, 2019
1,609
Oh that is an interesting trigger... @ericgibbs - it's functioning more like a differentiator with a threshold.

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#### Analog Ground

Joined Apr 24, 2019
305
The photodiode will have a reverse bias across it which means it is operating in photoconductive mode. Generally, this mode gives faster speed (higher bandwidth) but introduces dark (leakage) current. The other mode is photovoltaic where there is no bias on the photodiode. When using this circuit it is good to know in which mode the photodiode is operating.

#### Sergen Erbay

Joined Dec 19, 2019
20
hi,
It is wired as a Comparator, with hysteresis, R8 and R9 set the threshold voltage level.

E
What do you mean by threshold voltage

#### ericgibbs

Joined Jan 29, 2010
9,536
hi,
When using a comparator you need to have a Threshold voltage on one of the OPA pins against which the incoming signal is compared.
If the incoming voltage is less than the threshold voltage the OPA output is at one state, when the incoming voltage is greater than the threshold voltage the output of the OPA is in the other state.
OK.?
E

#### Sergen Erbay

Joined Dec 19, 2019
20
hi,
When using a comparator you need to have a Threshold voltage on one of the OPA pins against which the incoming signal is compared.
If the incoming voltage is less than the threshold voltage the OPA output is at one state, when the incoming voltage is greater than the threshold voltage the output of the OPA is in the other state.
OK.?
E
Thank you.I understood.

#### ericgibbs

Joined Jan 29, 2010
9,536
hi,
The threshold voltage on your circuit is approx +70mV, set by the 680k and the 10k from 5V.

E

#### Wolframore

Joined Jan 21, 2019
1,609
This is a very interesting circuit

See: