I am trying to understand exactly how to use transistors in simple circuits. From what I understand, transistors are primarily used for switching and amplification purposes.
I think I kind of understand the switching. Suppose we are talking about an NPN transistor. When a current is input into the base of the transistor this causes the transistor to turn on and will allow current to flow from the emitter to collector. How much current needs to be input to the base before the transistor turns on and will allow current to flow from the emitter to the collector?
For the amplification I am simply confused as to exactly how I would use one to amplify a signal. Suppose I have a signal. Where would I input this signal on the transistor (collector, base, emitter) and where would I get the amplified signal out? How much will it be amplified? Is this given in the data sheets for the specific transistors? Believe it or not I have actually had an electronics course, but I got so caught up in analysing circuits with transistors that I have become pretty confused. If I remember corectly the gain is relted to Beta which, I believe, is specific to each specific transistor so this will have to obtained from the data sheet.
I have been trying to wrap my head around these things by reading several different websites, but many of them seem to talk about how they work on the atomic level (n and p doping, etc) and don’t really seem to describe how I would use one. If anyone has any good websites on this I would be very grateful if you could post it. Thanks for the help guys!
The collector current will be in proportion (the ratio is the Beta) to the base current, Up to the point where the reisistance (load) in the collector circuit limits it. This is called saturation. Used as a linear amplifier you want the transistor about “half-on” with no signal, and then it goes from off to full-on as the signal varies over the input range.
Two rules: The base can’t be the output. The collector can’t be the input. The terminal that is not the input or output serves as the “common” return for both input and output.
If the emitter is used as an input, the collector is the output, the current gain will be 1, (actually (beta-1)/beta=alpha) but the amplifier can still provide an increase in voltage. How much increase depends on the ratio of source to load impedance. This is called common-base configuration.
Next, we can use the base as the input, and the emitter as the output. This is called common-collector, AKA emitter-follower operation. Voltage gain is 1, but large current gain (=beta) is possible. So it is possible to drive a low resistance load from a high resistance source…Significant power gain even if there is no voltage gain.
A more usual case is to use the base as the input, the collector as the output. This can provide current AND voltage gain. The voltage gain is approximatly Load R/SourceR * Beta.
a “real” amplifier will typically operate several stages in cascade, using various configurations. For example, a common emitter stage may drive a common base stage (“cascode” connection, dating to tube days) which then drives a common collector output stage.
Yes, the parameters are given, note that beta typically is very loosly specified. All the parameters vary strongly with temperature. Feedback techniques (AKA degeneration) are used to make the circuit gain insensitive to transistor parameters, and to aid thermal stability. This reduces the gain below the maximum possible.