Timeline for How does the compensation resistor in an inverting amplifier compensate for the input bias current?
Current License: CC BY-SA 4.0
16 events
| when toggle format | what | by | license | comment | |
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| Sep 21, 2020 at 12:00 | history | tweeted | twitter.com/StackElectronix/status/1308013130933587968 | ||
| Sep 18, 2020 at 22:55 | comment | added | Mahmoud Salah | @Circuitfantasist Yeah I noticed thank you :D | |
| Sep 18, 2020 at 6:24 | comment | added | Circuit fantasist | @MahmoudSalah, The assertion in your book, "The compensation resistor [R3] causes a current..." is wrong. A resistor supplied by a current source causes a voltage drop across itself. The op-amp inputs are driven by voltages, not by currents; so we should talk about voltages in the input part of the circuit. | |
| Sep 18, 2020 at 3:16 | vote | accept | Mahmoud Salah | ||
| Sep 17, 2020 at 17:13 | answer | added | G36 | timeline score: 9 | |
| Sep 17, 2020 at 15:52 | answer | added | Circuit fantasist | timeline score: 2 | |
| Sep 17, 2020 at 5:53 | comment | added | JRE | @MahmoudSalah: Just to be clear: Bipolar junction transistors (NPN, PNP) have a base, a collector, and an emitter. FETs have a gate, a source, and a drain. In your question, you mix terms for both types of transistor. | |
| Sep 17, 2020 at 5:50 | history | edited | JRE | CC BY-SA 4.0 |
added 2 characters in body; edited title
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| Sep 17, 2020 at 4:37 | answer | added | jonk | timeline score: 18 | |
| Sep 17, 2020 at 3:51 | comment | added | user1850479 | @MahmoudSalah Since the current through the gate of a transistor is only a few picoamps, FET amps often do not need a compensating resistor. However, bipolar transistors can have large currents through their inputs, so the resistor becomes important. Without it, current flowing through R1//R2 into the inverting input will generate a voltage drop which would be amplified. Adding R3 introduces an equal voltage drop at the inverting terminal, which cancels out. | |
| Sep 17, 2020 at 2:05 | comment | added | jonk | @MahmoudSalah I'm discussing bipolar, where these currents are far more worth discussing. There should be a current source/sink as part of the diff-pair. (I can't imagine how you've missed seeing it.) This current is split between the two halves of the diff-pair. Equally so if the inputs are at equal voltage (one hopes, anyway.) Given they are BJTs there will be a recombination current required for each half. This must be sunk (or sourced) through something. (Resistance, because that is what we are discussing.) So there will be a drop. A diff-pair can be either NPN or PNP. Not important which. | |
| Sep 17, 2020 at 1:40 | comment | added | Mahmoud Salah | @jonk but how there will be a collector current if the gate of this transistors are connected to ground are they a pnp transistor or what I am assuming it's NPN transistors but the explanation seems to be general for both, and my missing point is if I assume the diff-pair are of npn type then how there will be a gate current to the ground through this resistor in the first place ain't it supposed to be in the cutoff region ? | |
| Sep 17, 2020 at 1:16 | comment | added | jonk | @MahmoudSalah In bipolar, there are two things to worry about at the inputs: the bias current (needed because BJTs have recombination currents) and the offset current (due to the fact that no two BJTs are ever exactly alike in every particular.) Assume both inputs are equal in voltage. Hopefully, the collector currents in the diff-pair will be equal and therefore the bias currents will be equal. The problem comes if you use a 100 k resistor at one and a 10 k resistor at the other (both to ground.) The bias currents will develop different drops. That feeds back to an avoidable output error. | |
| Sep 17, 2020 at 1:08 | comment | added | Mahmoud Salah | No I didn't assume of course that there is only one transistor. and I saw the schematic and searched a bit before posting but thanks anyway | |
| Sep 17, 2020 at 0:57 | comment | added | Juan | you assume an op amp has a single transistor, I suggest you search for the schematic of a typical op-amp like a 741 and see what could be wrong in this line of thinking. | |
| Sep 17, 2020 at 0:41 | history | asked | Mahmoud Salah | CC BY-SA 4.0 |