How can a transistor be used as resistor and capacitor? I read somewhere that for the resistor, we can use a constant current source made up of transistors?


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    \$\begingroup\$ This question needs more words. Preferably ones that more clearly describe what exactly you intend to be asking. And/or links/hints to "somewhere" that "you read". \$\endgroup\$ – Asmyldof Oct 6 '16 at 15:57
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    \$\begingroup\$ Can you cite the source and quote the example of where somebody wrote that a transistor could be used to replace a resistor or capacitor? Most likely they were discussing the context of some particular resistor and capacitor in some specific circuit, but without seeing the context, we can't know for sure. \$\endgroup\$ – The Photon Oct 6 '16 at 15:57
  • \$\begingroup\$ A transistor can be used a base current controlled Rce resistance. The cap has nothing to do with resistance. Using a Base resistor, now the control is voltage controlled base current to control load resistance of collector . \$\endgroup\$ – Sunnyskyguy EE75 Oct 6 '16 at 16:13

Using a transistor in place of a resistor or a capacitor is sometimes done in integrated circuit design, because some processes don't offer suitable resistors or capacitors or they simply would take up too much area.

An example is shown below:

enter image description here

The transistor MC is a capacitor. The gate acts as one plate, the channel as the other. Source, drain and bulk are commonly tied together.

Transistor M1 acts as a resistor. It's simply a MOSFET operating in the linear region where it behaves as a resistor.

Another possibility to mimic a resistor would be a switched-capacitor circuit.

  • \$\begingroup\$ Thank you for your input. And yes it was something related to IC design,too \$\endgroup\$ – Osama Arshad Dar Oct 6 '16 at 17:11
  • \$\begingroup\$ Do you really think this is what OP was asking about? Particularly the part about whether we can "use a constant current source made up of transistors"? \$\endgroup\$ – The Photon Oct 6 '16 at 17:32
  • \$\begingroup\$ @The Photon - What other application would require a transistor to be used as a capacitor or a resistor? \$\endgroup\$ – Mario Oct 6 '16 at 17:36
  • \$\begingroup\$ @The Photon - Another possibility to use a transistor as a resistor would be in a bias circuit (= constant current source made up of transistors). But, the same techniques would be used. \$\endgroup\$ – Mario Oct 6 '16 at 17:45
  • \$\begingroup\$ Well likely what he read about replacing a resistor with a transistor was talking about a differential amplifier circuit, where they practically always first teach it with resistive loads, then replace them with current mirrors. I have no idea about the capacitor part of it though. \$\endgroup\$ – The Photon Oct 6 '16 at 18:13

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