# One transistor square wave amplifier: why doesn't it work?

I want to amplify a square wave coming from my signal generator using a single transistor and a 100V DC power supply.

This is the circuit I created:

(The orange component is a Volt meter)

I would expect the voltage across the 50 Ohm load to be 100V pulses, but instead the voltage is only 10.9V in the simulation when using a 12V square wave to the transistor base.

How do I amplify the 12V square wave from my signal generator to a 100V square wave using this setup?

• You have created an emitter follower instead of a common emitter amplifier.
– user16324
Commented Sep 29, 2020 at 20:21

Your circuit is wrong. You did this:

• So the transistor and the resistor needs to be interchanged
• you need a additional resistor to limit base current

Try it like this:

PS: the calculations are simplified and not 100% correct. but good enough to transport the principle

• Perfect example of how a simple change in drawing style and component arrangement can make everything much easier to understand. Commented Sep 30, 2020 at 9:27

You are doing high-side switching when you should be doing low-side switching.

Think about what voltage you want the emitter of the transistor to be. Now, think about your base voltage in comparison to this voltage. As you can see, you will have a hard time forcing current into the base-emitter junction if the base is 12 V and the emitter is nearly 100 V!

Now swap your load and transistor and see how this changes. With the emitter at ground and the base at 12 V, it should be much easier to activate this transistor. Keep in mind, you will need a resistor to limit base current in this configuration.

Always remember you need current flowing through the base for a transistor (BJT anyway) to work. That current has to go somewhere, and if the collector and emitter are both over 12 V, there is no way a 12 V signal at the base is going to be able to go anywhere.

You have built an emitter follower, and that is how they work. Switch the load to collector side, which seems to be what you want.