How to connect 2 circuits

[zesla]

Hey guys,

I have a critical problem with the electronics circuit when I design a circuit,
Actually I am not able to find out if I am right while connecting an input to the other output or not!
For instance when I design something by an opamp and want to connect its output to a transistor I am not sure if I have considered it just right, the matching impedances and so on? What to do? have to do some measurement or Just read the datsheet?

Any idea please?

 

[MikeMl]

You must understand Ohms Law and Kirchhoff's Laws, for starters.

For example, suppose you want to connect the output of an OpAmp to the base of a transistor. First, per the OpAmp's Data Sheet, it says that it can source/sink XXmA. There is a feedback resistor around the opamp so some of the output current flows in that resistor. The rest is available to drive the base of your transistor. Since the base of a transistor looks like a forward-biased diode, you must place a resistor between the OpAmp's output pin and the base of the transistor. The value of the resistor is a function of how much current is available, how much current you want to flow into the base, and what voltage will appear across the resistor...

 

[crutschow]

It takes a little effort to learn how to use, but I recommend an analog circuit simulation program such as LTSpice (free) to learn about circuits. You can connect many types of circuits and networks together, and observe how they work without every buying a component or touching a soldering iron (or blowing up a transistor). There is an LTSpice forum that will help with your questions about its use.