Author: Leonard Krugman
Although the manufacturer's data sheets for transistors are very useful in preliminary paper studies of circuits, it is often necessary to make direct transistor measurements. The block diagram of Fig. 3-8 illustrates the basic circuits for measuring the a-c open-circuit parameters r11, r12, r21, and r22. (Methods for measuring α and Ico are indicated later in this section.) The following general rules aid the experimenter in obtaining reasonably accurate results for all measurements.
- Use an accurate meter calibrated for the appropriate operating range. This is required since the transistor operates on comparatively small values of current and voltage.
- Measure the d-c bias voltages with a very high resistance voltmeter, to avoid meter-shunting effects. Shunting errors are particularly noticeable in the collector circuit which may have resistance of several megohms.
- Connect the test signal (usually 1,000 cps) through a step-down transformer that has an impedance ratio in the order of 500:1. This keeps the measurements independent of Rg and, at the same time, permits a low signal input without requiring a low oscillator gain control setting.
- Measure all calibrating resistors with an accurate bridge, or use a calibrated resistor decade box for the resistors.
- Check the waveform with an oscilloscope. The waveform quickly indicates reversed bias connections and overloads.