High speed Op Amp to NPN transistor design

[amartin32]

I (Novist) am currently building a analog circuit that consists of an LT 1056 op amp and NPN 2n2304 transistor. The objective is to vary the amount of current flowing through the LED (the 148 ohm resistor) proportionally to the input signal.

I have done this successfully with this circuit however now I would like to use this for input signals in the high kHz range to the low MHz range. The problem I have seen so far is the breadboard I used had a lot of parasitic capacitance and the 741 Op Amp how too low of a slew rate. Now that I everything seems find except when I go to 10kHz with a signal generator to oscilloscope. I see a phase shift on the output and a reduction in amplitude.

I think this could be due the Miller Effect? Their may be some stray capacitance at the junction of the transistors?

I computed the BJT load line for the operation of the NPN transistor and it shows we are operating in the linear region. (I was getting cut off before)

I appreciate any feedback as to find out why I get the amplitude reduction, phase shift and you have any design suggestion.

THANKS!

 

[Audioguru]

Why does your circuit have R8? It is not needed since the input of the transistor is a high impedance. Replace it with a short piece of wire and the circuit will work slightly better.
10kHz is not a high frequency. The opamp and transistor should not have a problem with it.

1) Measure the output of the signal source with your 'scope to see if the level of 10kHz drops.
2) Same at the output of the opamp and check for phase shift.
3) Same at the emitter of the transistor.

Do you know that the transistor inverts the signal with 180 degrees phase shift?

 

[amartin32]

Hi Audioguru, thank you for your response! I have removed R8, thanks.

I noticed the 180 degree phase shift (I don't know why? Could you kindly explain? I was using the invert function on the o-scope for this) but thats just switching the op amp from non inverting to inverting? Correct?

I will be checking all your recommendations today.

 

[Audioguru]

A transistor inverts (180 degrees phase shift) when the input is at the base and the output is at the collector as you have.
The opamp is shown to be a non-inverting amplifier.

 

[D.A.(Tony)Stewart]

GBW product of most Op Amps are low, so use unity gain.

ESR of LED is low ( 15 Ohms ) so emitter resistor should be low ( 100 )

Then to make constant current sink use feedback from emitter or FET source instead of OA output.

e.g. I would design this way
THen the BW is limited only by the Op Amp's max BW at low gain.

 

[chuckey]

Try putting a few 100 PF cap across R1, this will increase the gain at high frequency, like wise a small cap across R9. It would be better to take the negative feed back from R9, rather then the op amp output.
Frank

 

[FvM]

The maximum LED current which is supplied by the circuit in post #1 can be handled by LT1056 without an external transistor. D1 is for LED reverse voltage protection.