INTRODUCTION

This standard offers an easily-measured parameter which is one of the significant characteristics in determining the stability of a transistor intended for small-signal operation. The measurement technique allows rapid testing. Its close correlation to AC stability will help to establish the interchangeability of a device.

The symbol used for this common-emitter short-circuit feedback parameter is (C_re). The magnitude bars are included for three reasons:

(a) They reflect the true nature of the parameter in that it is an admittance magnitude rather than a susceptance.

(b) They eliminate the need for a minus sign. This prevents the confusion prevalent when the maxima of a minus quantity are to be considered.

(c) They reduce the possibility of mistaking the parameter for the imaginary portion of a reverse transfer matrix parameter (C_res for the Y-parameter or C_reo for the Z-parameter).

The parameter is expressed as a capacitance rather than an immitance because the latter is frequency dependent. It is desirable to allow easy correlation of the parameter with different measurement frequencies. (C_re) as defined and measured very closely resembles C_res (the short circuit Y-parameter) which is the reason for the subscript ‘'re".

It fits directly into the usual stability equations. A typical example is the equation for stability for a narrow-band amplifier with single-tuned interstages.

All of the terms in this equation except for(C_re) are either swamped by circuit constants or are relatively constant.

(C_re) is easily measured on relatively inexpensive equipment. It is a "plug-in-and-read" type of measurement. Because it is not a bridge measurement no adjustments or balancing are required.

(C_re) is a measurement on an active, full-biased, transistor. In this it differs from C_cb which is defined for zero emitter bias current.

It is a three-terminal admittance measurement. Thus it doesn't fall heir to the numerous difficulties and inaccuracies of the two-terminal measurement, C_ob.

The low frequencies used enable the measurements to be more easily reproduced. Correlation is improved. It is granted that the measurement of feedback at the actual operating frequency is the more efficacious method for determining stability performance; but the intent of (C_re) is to provide a suitable means for judging transistor interchangeability over the widest possible range of transistor types, applications, and frequencies.