single supply op amp problem

Discussion in 'The Projects Forum' started by volvo340, Feb 1, 2011.

1. volvo340 Thread Starter New Member

Feb 1, 2011
4
0
Hello, first time on the forum, any electronics forum in fact so thanks to any one who can make any suggestions.

I have the op amp circuit below. It's a simple non-inverting LM358 circuit.
Vin is supplied by the output of a bluetooth receiver which outputs about 0.6 volts when the bluetooth receives a signal. The current output seems to be very low, maybe 20 microamps (too small to read on my meter). I took the BT receiver from another product which has further circuitry to drive some LEDs.

I want to use the output of the receiver to switch a transistor used as an electronic switch. So I want to saturate the transistor so that it can pass 200mA (which is the current drawn by the load I want to turn on and off using BT). So it will need some amplification (current and voltage) between the BT receiver and the input to that transistor switch and also some buffering. I hope to acheive these things with this op amp circuit.

SO the problem.With the circuit values as shown I get a Vout of 1.2V, which is what I expect since gain = 1+R2/R1. When I increase R2 to 33k though I only get an output of 1.5v. in fact 1.5v is the maximum output whatever values of R I choose.
(Rin in included because if it's not there then the opamp output goes to around 4.5v).
(The 6V is supplied by 4 AA batteries)

• op amp 2.GIF
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Last edited: Feb 1, 2011
2. nigelwright7557 Senior Member

May 10, 2008
488
71
I think you are going to need a rail to rail op amp for this project.
Look them up....

3. wayneh Expert

Sep 9, 2010
12,381
3,236
I think your input impedance is too high with that Rin load. Try a 1 MΩ or even higher.

Are you saying it CAN go to 4.5v (by removing Rin), but cannot go above 1.5v with it attached? That's good, that the amp can output that voltage when it wants to. Otherwise that might be a concern, that your amp is toast.

4. volvo340 Thread Starter New Member

Feb 1, 2011
4
0

Sorry I don't understand that. If you say 100K too high already, then wouldn't a 1M make it even higher?

Yes, the output goes to 4.5 volts unless the input (output of the BT device) is grounded through a resistor.

5. volvo340 Thread Starter New Member

Feb 1, 2011
4
0

Why do you think that?

6. wayneh Expert

Sep 9, 2010
12,381
3,236
I meant the load is too high, resistance too low. I consistently get those turned around, so do as I do, not as I say.

7. wayneh Expert

Sep 9, 2010
12,381
3,236
Even when the BT output is off? Are the two circuits well grounded to each other? You may be looking at an output that goes open (and up) when off, and opens a path to ground when the LED turns on. The 0.6v would be the voltage across the output transistor.

8. thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Try wiring it like This Circuit

If you are looking for voltage gain.

9. volvo340 Thread Starter New Member

Feb 1, 2011
4
0

I'll give that a try.

Yes, the output of the BT Rx is connected to the ground of the op amp supply and everything works exactly as expected up to a gain of two.

I'll try supplying the circuit from another source to and see if that makes a difference to isolate the cause of the problem to the BT output. I already know that out drops when connected to the base of a transistor directly which is why I was hoping to buffer it.

Grateful for the suggestions everyone, thanks a lot.