DC Transistor voltage amplifier

[Froskoy]

Hi,

I like a very hands on approach to electronics and am trying to amplify a voltage from a photodiode (TSL251) using a transistor.

I was just wondering if anyone had a suitable circuit or could point me in the right sort of direction?

Many, many thanks,

Froskoy.

P.S. I'm also trying to create a buffer circuit using a transistor - is this page a good place to start?
Basic Buffers

 

[shimniok]

I tried this with an op amp set up non-inverting with just the right amount of gain to get voltages in the 0-5V range or so. It more or less worked, but would've been nice if I'd set up the circuit so the minimum voltage of the photodiode produced 0V output -- or something much closer to 0V than I was getting.

I've no doubt there are far better ways... (it's not like I know what I'm doing after all ) am curious to see if anyone posts up ideas on how to do this.

I am guessing that part of the trick is in understanding the electrical behavior of a photodiode... how much current does it put out, how much voltage, what happens with a load, etc... ?

Wished I could be of more help.

 

[audioguru]

A photo-diode can be used in two ways:
1) With a reverse bias voltage it leaks a small current when exposed to light.
2) With no bias voltage it generates a small voltage and current like a tiny solar cell.

A transistor needs a high base current. Use a high input resistance non-inverting opamp circuit as an amplifier instead.

 

[atferrari]

A transistor needs a high base current. Use a high input resistance non-inverting opamp circuit as an amplifier instead.

What does high mean in this context, Audioguru? You are comparing it with what?

 

[Froskoy]

Yes, I'm currently using an opamp and it works perfectly. This is purely for my own interest.

I really want to experiment using an application of amplifying a small voltage (eg from a photodiode) using a transistor of some sort (yes amplifying voltage) - purely for my own interest. Any suggestions?

Many thanks,
Froskoy.

 

[audioguru]

What does high mean in this context, Audioguru? You are comparing it with what?

We are talking about the extremely small current from a photo-transistor. The fairly high base current of a transistor is almost like a dead short to it.
Maybe if you add a second transistor or more to make a darlington transistor or make your own opamp that the photo-diode will not be loaded too much.

 

[Froskoy]

So amplify the current using a transistor first and then do the voltage?

Sorry, I've never done any work with transistors before.

 

[audioguru]

You can use one transistor an an emitter-follower to amplify the current then use a second transistor as common-emitter to amplify the voltage.
Or you can use an opamp to do both.

 

[Froskoy]

Great stuff audioguru. Thanks very much for your help - it's really appreciated.

One last question - please could you suggest part codes for each transistor? Many thanks.

 

[audioguru]

In Europe the BC547 transistor is popular. I am in Canada and I use the 2N3904 transistor that is almost the same except its pins are reversed.

 

[colin55]

We are talking about the extremely small current from a photo-transistor.

This is an entirely wrong way of considering the situation.
A photo transistor is placed in a situation where the device changes resistance according to the light falling on it.
Place the photo transistor between base and positive rail of a 6v supply and use any small signal transistor with 2k2 to 10k load resistor.
The output voltage (from the collector) will be HIGH when no light is detected and very near 0v when light is detected by the photo transistor.

 

[audioguru]

Guess what?
The TSL251 is not a photo-diode and it is not a photo-transistor.
It is a complete IR receiver with a photo-diode, an opamp and a very high value feedback resistor.
It is very sensitive and with a 5V supply its output goes from 3mV (dark) to 3.5V (lighted).
It is spec'd with a 10k load.
This is from its datasheet:

 

[Froskoy]

Thanks everyone - I now have a good idea of what I want to do.