H-bridge simulation

I always believed that to build an H-bridge I needed to use FETs. I'm guessing that it was because of the small device package of Bipolar Transistors that I thought that it will not be able to power it. Well today I realized that some of my motors are probably small enough (low power) to be powered by BJTs. This also made for a great opportunity to test LTSpice some more. Eventually I hope to improve my mobot H-bridge design in efficiency using these tools.

Below are the results. You can see that the flyback diodes really do help protect by limiting the various potential differences to below their respective breakdown levels. The blue trace represents the input signal and the green trace is the voltage at one end of the inductor with respect to ground. 

Playing with Darlington Pairs

I wanted to light an LED without affecting the input impedance of the device I was measuring without buying a FET. Having the Darlington pair creates the digital-like steep ramp you see below in the simulation results by achieving the product of the current gains (beta) of each of the NPN transistors. The base resistor decreases the base current to a minimum to prevent affecting the load of V2. The green line shows the I-V relationship of voltage source V2 and its current output. You can see a slight change in slope (Ohms) at the output end, but it is negligible considering the purpose.

Back to Basics

After briefly reading my textbook, I decided to build a current mirror using random BJTs that were lying around. Q3 is there to reduce the base-current error. Since I don't have a desktop current source I decided to use a biased PNP transistor. This was a great opportunity for me to try the simulation capabilities of LTSpice. As you can see below, the LTSpice simulation closely matched the actual data.

Strangely, regardless of Q3, there was a 33% loss in the current mirror. This was probably because of the low current level I was mirroring. High beta values lead to higher error as well.