I was asked to analyze this Op-amp circuit and I need a little bit of advice about it. I have presented my analysis about the circuit, kindly correct me wherever I am going wrong.


simulate this circuit – Schematic created using CircuitLab

  1. If a positive voltage is applied at Vref then the negetive terminal should also necessarily have its voltage at the same.
  2. Hence a current will try to flow through the 100 ohm resistor into the ground, but will be unable to do so, since there is no circuit path. What I don't understand is what will happen in this case, will there be a current flowing through the amp feedback short path?
  3. If the reference voltage is negetive, a current may flow through the emitter, but what will be the voltage at the base? And even if a current flows through the emitter, will the reverse biased LED through the collector allow this.

Hence, I think the circuit is wrong, please give me your suggestions...

P.S. The diode D1 is a laser Diode, I don't know what that implies. Is it different from an LED diode?

  • 1
    \$\begingroup\$ I'm assuming the transistor should be NPN instead of reversed PNP? \$\endgroup\$
    – s3c
    Commented Jun 26, 2014 at 9:06

1 Answer 1


You can simulate this with falstad.com/circuit and clearly see what happens I've build it already for you just play with the slider for Vref and see what happens.

When Vref = -1V then there has to be -1V over (-) of the opamp to get this the opamp's output has to be negative so that a basis current can flow this current comes from the emitter the colector current will be higher and the led will turn on

When Vref = +1V then Vemmitor should be 1V but this can never be achieved so the opamp's output will be (+1V-(0A))* ampfactor => opamps max saturation voltage So the transistor basis is high and there won't be any current flowing through the led

you can see it like this, in this particular circuit enter image description here

  • \$\begingroup\$ Okay, but what I don't understand is that why is the transistor drawing current inwards at the collector. Shouldn't the current direction be outward at the collector of a pnp. Or else how would the relationship Ib+Ic=Ie stand? \$\endgroup\$
    – Ghosal_C
    Commented Jun 26, 2014 at 10:09
  • 1
    \$\begingroup\$ it's more something like this in that circuit ibiblio.org/tkan/audio/bjt2.png \$\endgroup\$
    – Bruce
    Commented Jun 26, 2014 at 10:15

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