bad design?

I was going to use an opamp…

Then thought why not a couple of transistors! So I went searching and found this (attached) the emitters voltage will be that of the base pin - foward voltage drop, I can’t see this working but I’ll build it and see for myself!

I think that the circuit should work.

I plucked a figure from the air of 10mv pp… then began to amplify it using transistors, fat chance that circuit will work with such a small voltage… unless of course a reverse diode produced > 10mv eg if 100mv up to a volt can be produced then sure…

After some experimenting I came up with this which amplifies s 10mv signal.

Not tempted to use an opamp?

Johnny010: Not tempted to use an opamp?

For simplicity ... yes :)

Not sure what the problem is here.

A (reverse-biased) photodiode passes current - not the same as a photovoltaic generating a voltage - when illuminated. The transistor amplifies that current. What is the problem (apart perhaps from lack of current gain)?

Paul__B: Not sure what the problem is here.

A (reverse-biased) photodiode passes current - not the same as a photovoltaic generating a voltage - when illuminated. The transistor amplifies that current. What is the problem (apart perhaps from lack of current gain)?

Yes lack of current gain....

I spent ages looking for the kind of voltages produced by an ir receiver when detecting an ir signal... it could be 1 or 1000mv for all I know so I started tryinh to amp a 10mv pp signal...

Obviously an opamp is the way to go.

If it is lack of current gain, just use a Darlington.

In this application at least.

Paul__B: If it is lack of current gain, just use a Darlington.

In this application at least.

I don't think a darlington would cut it either, (I'm sure I tried a darlington pair) hence the need for feedback from the collector to base..

Opamp would be a very simple and neat package. 1mV is below most opamps minimum voltage offset mind. The LM324 is not bad and is dirt cheap:

  1. 3mV typical offset detection.

2. 1MHz Unity Gain Bandwidth (assuming you want say a 1,000 gain so 3mV->3V 5mv->5v etc.)...

If you use one op-amp with a gain of 1000, that would give you a total bandwidth allowance of 3KHz.

Using 2 for a gain of 900 in total (gain of 30 on each), you can get 65Khz ish max bandwidth.

3 of the op-amps in a cascade with a gain of 10 on each, you will have a total bandwidth allowance of 152Khz.

  1. They are cheap! Take up less space. Easier to debug instead of trying to find the faulty component.

http://www.ti.com/lit/ds/symlink/lm324.pdf

I will..... just curious .

The first circuit is just an IR detector. It just lights an LED in the presence of sufficient IR, no matter what it is.

The circuit you simulated is nothing like it. So I'm not sure it proves anything.

polymorph:
The first circuit is just an IR detector. It just lights an LED in the presence of sufficient IR, no matter what it is.

The circuit you simulated is nothing like it. So I’m not sure it proves anything.

But does it really? (Waiting on the ir receiver diodes) how much voltage is produced from an IR strike?

This blog post may be of interest: Poor maker’s Infrared receiver #2 http://www.analysir.com/blog/2014/05/28/poor-makers-infrared-receiver-2/

Note: Using an IR LED, which are typically fast switching (~ns) you should be able to capture the full modulated IR signal. However, if you use an IR phototransistor, which typically have slower rise/fall times, you should get the demodulated IR signal passing thru (based on some recent tests).

Photo diodes are often used in reverse biased mode, where IR radiation causes
current flow - voltage is irrelevant, as you feed the current into a current->voltage
conversion circuit such as common-base BJT or opamp or specialised photo-detector
IC.

This is the most sensitive and fast method of using photodiodes, and the output is
linear in the radiation intensity too.

For instance:

What MarkT said. And AnalysIR.

Don Lancaster ( www.tinaja.com ) won a patent lawsuit against Bell Labs over the use of an LED as both a transmitter and a receiver for use in fiber optic lines.

Phototransistors are bad news with fast signals. A PIN photodiode or an IR LED work much better.

Again, your simulated circuit isn't even similar.

Similar? It's not supposed to be...

If I created a 10mv peak to peak, the first circuit fails! (Back to working out how much voltage /can/ be produced from say a remote control .

Speed is not an issue, amplifying it without an opamp was hence my circuit (2nd circuit)

Maybe you should tell us what you are trying to accomplish.