18-05-2017, 02:42 PM
(This post was last modified: 18-05-2017, 02:43 PM by ppppenguin.)
Sorry, Jim. I must disagree. Let's also not compare apples and oranges. Common base inherently has a current input. As does a common grid or common gate stage. In theory an ideal common base amplifier has a zero input impedance and so you wouldn't attempt to drive it from a voltage. Unless you use a resistor as a voltage to current converter.
So referring to common emitter configuration only. The small signal collector current is defined by the base voltage according to the well known formula for mutual conductance. For better or worse it's highly dependent on the DC collector current so with large signals a common emitter amplifier is inherently non-linear unless you apply -ve feedback. https://en.wikipedia.org/wiki/Transcondu...ransistors
A common emitter stage with current feedback (resistor from collector to base) is rather like a crude opamp. You're creating something approaching a virtual earth at the base. But the current gain is set primarily by the resistor, not the device. As has been said before, the beta of a device is utterly unpredictable and in an ideal world would tend to infinite. Apart from the fact it would upset the bias you can do exactly the same with a valve or FET, both of which have infinite current gain because the grid/gate current is near enough zero.
Out of curiosity I dug out my copy of Horowitz and Hill (1st edition). It's unfortunate that they introduce the transistor as a current amplifier (hfe) and only later bring in the Ebers-Moll transconductance model. To give them credit, as soon as they've introduced hfe (aka beta) they say that it's not a "good" transistor parameter becuase it's so variable. To quote them: "A circuit that depends on a particular value for hfe is a bad circuit". They also say in their intro to transistors that the current gan model is only introductory and they will move on to Ebers-Moll.
So referring to common emitter configuration only. The small signal collector current is defined by the base voltage according to the well known formula for mutual conductance. For better or worse it's highly dependent on the DC collector current so with large signals a common emitter amplifier is inherently non-linear unless you apply -ve feedback. https://en.wikipedia.org/wiki/Transcondu...ransistors
A common emitter stage with current feedback (resistor from collector to base) is rather like a crude opamp. You're creating something approaching a virtual earth at the base. But the current gain is set primarily by the resistor, not the device. As has been said before, the beta of a device is utterly unpredictable and in an ideal world would tend to infinite. Apart from the fact it would upset the bias you can do exactly the same with a valve or FET, both of which have infinite current gain because the grid/gate current is near enough zero.
Out of curiosity I dug out my copy of Horowitz and Hill (1st edition). It's unfortunate that they introduce the transistor as a current amplifier (hfe) and only later bring in the Ebers-Moll transconductance model. To give them credit, as soon as they've introduced hfe (aka beta) they say that it's not a "good" transistor parameter becuase it's so variable. To quote them: "A circuit that depends on a particular value for hfe is a bad circuit". They also say in their intro to transistors that the current gan model is only introductory and they will move on to Ebers-Moll.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
