LT Spice For Radio Amateurs: PART 4

So lets bring everything we have done so far together into 1 project. So we use a new component in this schematic a transistor. To add the transistor select components->npn. The we can right click the component and select a transistor type. For this circuit we will use some rather standard 2n2222 and the 2n2219a.

So in the above circuit we have a voltage source V2 where you will notice the proper use of Label Nets. Anywhere a Label Net has been placed with the value of 12v, the 12 volts fromĀ  V2 will be applied. Run a transient analysis with the following values to see for yourself how the Label Net works “.tran 0 0.000001 0.0000001”.

 

Ok, so what is the above circuit. It is a rather poorly designed buffer and amplifier. What is does however is demonstrate everything we have done so far. We have voltage and signal sources, we have the voltage divider R1 and R2, we have DC Blocking caps C2, 3 and 5, we have L1 blocking AC from entering the power rail. Something else you might notice is we are using 200 ohms for the load and source impedance.

So, once you have the circuit built, run a simulation and follow the signal through the circuit. There is so much that can be learned from probing about the circuit. Voltages can be found on the base, emitter and collector of the transistors, the current flowing through the transistors can be found and we can see the amount of total gain of the amplifier.


So for those who have been paying attention, you will have noticed that the input signal is 0.2v p-p, the green trace in the above circuit. And the output at the top of R6 is close to 5v p-p, which means that the gain is close to 27db.

And there you have it, you now know enough to be able to make complex circuits in LT Spice and simulate them and make usable measurements to define some parameters of amplifiers.

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