Input and output impedance - Page 1
Forum section: electronics / beginners

author: goldcoin
author_rank: Contributor
timestamp: July 27, 2015, 06:10:22 am
content: How do you calculate the input and output impedance for an electronic circuit (in general) ?

author: Simon
author_rank: Unknown
timestamp: July 27, 2015, 06:46:36 am
content: It's called thevenin's theorem:https://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem

author: goldcoin
author_rank: Contributor
timestamp: July 27, 2015, 07:13:30 am
content: Quote from: Simon on July 27, 2015, 06:46:36 amAnd if there are transistors in the circuit we replace them with small-signal/Ebers-Moll models and then apply Thevenin's theorem?

author: tron9000
author_rank: Frequent Contributor
timestamp: July 27, 2015, 08:09:46 am
content: http://wiki.analog.com/university/courses/electronics/text/chapter-9#output_impedance_current_follower_or_common_basegate_amplifierexample abovegoogle is your friend...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 27, 2015, 01:58:58 pm
content: If you mean, of an arbitrary, truly general circuit, there need not be any single figure, nor does it need be computable at all!Even for simple circuits, it can vary with voltage, time, internal configuration or state, etc. And giventhoseconsiderations, there are many (also non-unique) methods of averaging it, to get an equivalent value.A classic example being an RF amplifier: the output capacitance of a transistor varies with voltage (both the supply or bias voltage, and the signal voltage), so that the matching impedance varies with voltage as well. The best you can do is match the average (given a suitable averaging method), but even that is not very helpful, because it is only valid for one particular level.If you have a fairly well-behaved, but otherwise unknown, circuit in a box (a "black box" experiment), and wish to measure its characteristics, then you can use a number of theorems to find this: you can use different load resistors or impedances to calculate the Thevenin equivalent, or the Norton equivalent, or a number of matrix-oriented methods that arise from the study of simple, abstract amplifier circuits or RF systems (e.g., the h, z and s parameters).Tim

author: goldcoin
author_rank: Contributor
timestamp: August 03, 2015, 01:09:36 pm
content: Quote from: tron9000 on July 27, 2015, 08:09:46 amSo in case of a common collector amplifier the input voltage is between the collector and base and the output voltage is the voltage drop between the collector and emitter ?

author: LvW
author_rank: Frequent Contributor
timestamp: August 03, 2015, 01:33:45 pm
content: Quote from: goldcoin on August 03, 2015, 01:09:36 pmYes - in, principle you are right. However, your wording sounds a bit "uncommon".I think, it is more clear to say:In a common-collector stage the collector node is grounded. Therefore,* the input is between the base and ground,* the output is between emitter and ground.For measuring/simulating the corresponding resistances you simply need the corresponding voltage-to-current ratios.Please note that for determining the output resistance it is necessary to inject a test current and to measure the voltage drop. And don`t forget to ground the signal input!

author: goldcoin
author_rank: Contributor
timestamp: August 04, 2015, 04:51:03 am
content: Quote from: LvW on August 03, 2015, 01:33:45 pmThe common collector schematic shows the emitter resistor connected to ground and the collector connected to the positive voltage source. So the input is between base and emitter resistor and the output is between emitter and emitter resistor. That would mean that the emitter resistor is common to both input and output ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2015, 05:26:04 am
content: When speaking of AC signal analysis, supplies are ground. The term used for this is: a supernode.Tim

author: LvW
author_rank: Frequent Contributor
timestamp: August 04, 2015, 07:49:16 am
content: Quote from: goldcoin on August 04, 2015, 04:51:03 amThe term "common collector" means:The collector is a common node for defining input and outputsignals- and, as mentioned by T3sl4co1l - this applies to ac only (not DC). Therefore, the supply voltage represents an ac ground (zero source resistance).