Curve tracer example 3:

Voltage regulator measurements

Updated December 10, 2005

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How do you know when a packaged amplifier contains a linear voltage regulator if you don't have the schematic? You take it to your curve tracer!

In this example we will assume you own a Tektronix 370B curve tracer, the best there is. However, the procedure can be adapted to any model curve tracer if you will put on your thinking cap for a few minutes. Always start by disabling the output of the curve tracer to your DUT, and setting the collector voltage to zero (it's a big knob, you can't miss it). Turn the collector supply knob all the way counterclockwise to set it to zero.

In this example we will assume that you amplifier is not a dual-biased part (has V+ and V- terminals) that will be unhappy if it sees the positive supply without the negative one present. The specific example was a crusty 20-year old amplifier from "California Amplifier", model C30622 (we checked, it isn't listed on their web site, and they seem to have morphed into something other than an amplifier company). This unit has a single DC input labeled "+15V", and that is what we are going to look into.

Connect the amplifier supply terminals to the curve tracer (we'll abbreviate it CT) this way:

V+ connection of amplifier=collector of CT
No connection=base of CT
Amplifier package ground=emitter of CT

Cycle the 370B on/off to clear out whatever settings that last person used. Although the 370B defaults to very benign conditions, you should make a habit out of checking a few things we will describe here; if you have an older CT, you will need to manually set more of the control knobs. You should check to see that the polarity of the collector supply is positive (set to NPN on old curve tracers) and it is in swept mode as opposed to DC or AC modes. You won't be using the step generator, so you don't have to even look at any of its controls.

Now set the knobs on the CT this way (order is not important, but there are five knobs you need to adjust):

1. Set vertical/division to 20 mA (this is merely a guess, unless you know apriori how much current the amp will draw).
2. Set horizontal/division to 2 Volts
3. Set collector supply to "emitter open"
4. Set peak power to 10 watts (it will take some power to turn this thing on!)
5. Set maximum collector voltage to 80 volts (you'll never get to 15 volts if set it to 16 volts, because of the series resistor that limits the power)
6. Be sure that the collector polarity is set to positive (this is the default when you turn on a 370B CT, but get ion the habit of checking this it will save your from disaster some day)

Now you are ready to begin the measurement.

1. Enable the outputs that are connected to your DUT (you could be plugged in to either the left side or the right side of the CT).
2. Increase the "collector supply" until you see the I-V curve. Looks like we guessed right on the 20 ma/division on the vertical axis, but if the trace goes off the screen now is the time to adjust it. Stop when you get just past 15 volts (it's probably not gonna explode if you go to 16 volts) In this case, the display looked like this:

What can you tell from the curve? A lot! This amplifier model definitely contains a regulator. You can see where the regulator "cuts in" around 1.5 volts, less than that and it acts like an open circuit (a horizontal line). This "overhead" must be exceeded before the regulator outputs any current, and it is usually quite close to the regulator's minimum drop-out voltage. Around 10.5 volts the current abruptly stops climbing with voltage, this is where the regulator finally starts regulating. If you subtract the overhead of 1.5 volts from the the point where it begins regulation, this is a good indication of the voltage output of the regulator. In this case the amplifier contains a nine-volt regulator.

The "looping" is due to a large capacitor on the input, probably n the order of 1 microfarad. This is a good thing, voltage regulators are not usually stable without a microfarad or two on input and output.