For my last design question, Transistor current control with collector feedback, I want to add a NPN transistor between OpAmp and already shown transistor in common emitter mode. With new design, the base of output transistor (already included) will be driven by newly added NPN transistor as collector load.

My question is: To design the values of RE and RC of newly added transistor, which factors do I need to consider ?

What I already know is only that the total resistance of new Transistor should be adjusted such that it's base current Ib does not exceed the output current limit of OpAmp----- Ic = 15 / R and Ib = Ic / hFE

Now how do I adjust this calculated current limiting resistance R between Collector and Emitter of a new transistor. As far as I know, Emitter Resistance is also feedback to input voltage.

The circuit operates in linear mode.

Thanks in Anticipation for your time and opinion.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ A schematic would really help here...... \$\endgroup\$ – Trevor_G Apr 19 '17 at 11:41
  • \$\begingroup\$ I'll go Trevor one better - without a schematic, there is no way to answer your question, since there is no way to be certain what circuit you are proposing, and no way to determine what your resistors actually do. I can guess, but I don't know. So edit your post and use the schematic icon (the one with the diode), or hit ctrl-m. \$\endgroup\$ – WhatRoughBeast Apr 19 '17 at 12:25
  • \$\begingroup\$ @WhatRoughBeast \$\endgroup\$ – HerrderElektronik Apr 19 '17 at 13:11
  • \$\begingroup\$ The new transistor inverts the signal. What does that do to your feedback? \$\endgroup\$ – owg60 Apr 19 '17 at 15:11
  • \$\begingroup\$ That circuit will not regulate current. It will regulate voltage at one end of the sense/load resistor. Any change in supply voltage (including ripple and noise) will be reflected directly in the current. Also, in your linked circuit, you have missed the fact that the load is normally connected to the collector of the NPN. As owg60 comments, your feedback polarity is wrong. Finally, the base of Q2 will force the Q1 collector to a range of 0 to 0.7 volts or so, so it won't work the way you think it will. \$\endgroup\$ – WhatRoughBeast Apr 19 '17 at 15:37

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