Computing stuff tied to the physical world

Easy Electrons – Transistor circuits

In Hardware on Jan 22, 2011 at 00:01

With the basics and some real-world details out of the way, it’s time to start looking at some other circuits with transistors.

I’m going to describe three different types of circuits, to give you a feel for transistors – while noting that I’m just scratching the surface w.r.t. these amazing devices:

• regulated power supply
• more current with chained transistors
• push-pull drivers

Regulated power supply

In the post about diodes, I described a circuit based on a zener diode. One problem with that circuit was that it only worked for a few milliamps of current, the other was that this current was being drawn even under no load.

With a transistor, we can fix both problems at once:

Well, we’re not changing the fixed current draw really. But we can use a higher resistor value to draw less current, since only a little bit will will be needed to feed the transistor base.

So what’s going on here? Well, remember that the B-E junction is essentially a diode, connected in the forward direction. So from base to emitter, we have a 0.7 volt drop. Since the zener keeps the voltage fixed, the output voltage will be 0.7V below the zener breakdown voltage: i.e. 4 – 0.7 = 3.3V, tada!

The neat thing is that this is basically all there is to it!

The output voltage doesn’t really depend on the load current. With an amplification factor of 100, with a very light 1 mA load, the transistor will need to draw 10 µA on its base to generate that amount of current. It may seem like it does this as if by magic, “seeking” the proper settings all by itself. Let me try to describe what happens anyway. What we’re doing is keeping the base voltage at a fixed level. Here’s what happens when things start up, i.e. when the emitter is still at zero volts, right after turning on the power:

• the base will then be at 0.7V, because it’s always 0.7V above the emitter level
• this is much lower than the zener voltage
• so all the current through R1 goes into the base
• the transistor will amplify this and start conducting
• that will “pull” the emitter voltage up, since the collector is tied to the input voltage
• this in turn means that the load will be supplied with power
• assuming the load is not a short circuit, the voltage on the emitter will rise
• so will the base voltage – by 0.7V more, as always
• at some point, the base reaches the zener voltage
• that’s when current will start going into the zener
• meaning: less current into the transistor base
• where does it end? simple: when the emitter is precisely at 0.7V under the zener voltage
• this adjustment process takes place continuously, so when the load current changes, the transistor will keep on adjusting its current to keep the base at the zener voltage
• similarly, this will keep the emitter voltage at a fixed level even when the input voltage changes, since the voltage at the base remains the same

Voilá, a regulated power supply with a constant voltage output, a.k.a. a linear voltage regulator.

This voltage regulator will work fine for fairly large currents. After all, if we have a 10 mA current through R1, then a transistor with hFE 100 will be able to amplify this up to 1A.

But there is a catch: power consumption – again!

Note that the collector is held at the input voltage, and let’s assume this is fixed at 5V. And the emitter is held fixed too, at 3.3V in this example. So we have 1.7V on the C-E junction, with all the load current going through it. With a 1A current draw, that’s 1.7 watt of power. Not huge, but already well beyond what a little transistor can handle. This is why such circuits need to use power transistors and large heat sinks.

It gets worse with higher input voltage. Suppose we connect our setup to 9V instead of 5V. Now the C-E junction will have 5.7V over it, while still drawing 1A = that’s 5.7W of power, i.e. generated heat! So keep in mind, no matter how fancy your regulated supply is: if it works according to this “linear” principle, then it’ll turn all excess voltage into heat. See also an earlier post about this topic.

Our circuit also gives us a new way of looking at a transistor: while regulating, as shown above, it acts like a variable resistor, with the resistance being controlled by the amount of current flowing into the base.

So there you have it, the basic properties of a linear voltage regulator. I don’t want to stray too far from microcontrollers in this Easy Electrons series, but this really is a good example of how unusual transistors can be, compared to resistors and capacitors. Besides, voltage regulation is so common that it’s really useful to understand how it works and to be able to reason a bit about power consumption.

Tomorrow, we’ll go into “Darlingtons”! :)

1. A small nit that might confuse somebody: So we have 1V on the C-E junction,…. That should be 1.7 V.

Thanks for these articles, by the way. So far it’s all been a refresher for me though somewhat useful as such. I look forward to you getting on to FETs as I have no practical experience of those.

2. Awesome tutorial