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Can someone explain how does a voltage controlled current source work using transistor and op amp? Does anyone have circuitry to construct this?Thank you.
I have attached a simple schematic of a voltage controlled current source. The analysis is straight forward. We have an op-amp configured as a follower and the negative op amp terminal attached to the source of the transistor. Since the op amp will have a very high open loop gain at DC the negative input termonal will see the same voltage as the positive terminal, Vin. Therefore the voltage presented across the resistor is the input voltage which you can vary. By selecting a sensible value for the resistor knowing the input voltage range that you wish to apply you can then set the output current range. So for an input range of 1V to 5V and a 1K Ohm resistor you can vary the output current from 1mA to 5mA.
Thanks for the input. I understand the schematic that you gave me. The load should be put in between the emitter and the negative terminal of the op amp. However I can't figure out how I can put my load since it consist of transistor and resistor. My goal is that I'm going to measure the switch on resistance of the transistor. and by the way the transistor is inside the IC and the only pin that I have access is the collector pin. Thanks again.
I am not clear on what you are trying to achieve - are you able to send me a schematic of how you intend to set up the op amp and the bipolar transistor with any loads attached including your power rails? If you are using a discrete bipolar device you should have access to all 3 output pins on the package as its a TO-18 metal can right? Or are you stating that the bipolar transistor is within the same package as the Op Amp?
Maybe you could include the data sheets for the components your are using also? I will have a look tomorrow.
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