Tuesday, December 4, 2018

picking number from thr air vs designing

I will try to explain how I design the amp with little math and datasheet supplied data. First I pick a transistor. For this example I will use the KSP10.
The three things to look at are voltage, ß or hfe, and frequency. I will be making a battery operated amp so voltage is good. My frequency is 2 MHz so good here too. ß is 60. So how to use this info?
This is the circuit I will use. Now for more assumptions. I want Zin and Zout to be 50 Ohms. I want Vce = 2 volts. This is not the best of amps but will help with the understanding of my methods.
I don't like high current drain circuits. With a 3 volt supply and 50 Ohm load resistor I will be dropping 1 volt across R1. The current required is 20 ma. Knowing ß is 60 allows me to size the feed back resistor so R2 is 3250 Ohms. I run the sim and find Ir1 = 20ma. So my Q current is good. It is fairly simple and the circuit is 100% stable. I can hear someone ask how? The base voltage depends on the collector voltage which depends on the base voltage. Going in circles like that creates negative feedback. If you drive the collector to saturation you cut off the base bias which pulls the collector out of saturation. Neat huh?
Hear I check the collector voltage. I was looking for 2 volts. Looks good.
As I have said many times before get the DC set and then add AC components. This is the frequency response and gain I get.
Here are the signals on the transistor. The red and blue are the transistor base and collector. As you can see they are out of phase and this makes for a stable circuit. The gain is rather low so what can be done about that?
As I said before the AC impedance and DC resistance are different. Here I added a coil to 'boost' the AC impedance to increase the circuit gain.
 The coil value would depend on the desired frequency. I changed the coil value to see the effect.
I changed the output to see the effect.
This a a low Z circuit so draws high current and does not have a very high gain. Next time how to reduce current and get higher gain.

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