Electronic Design

Extending High-Frequency Response

High-frequency transistor limits can be extended and circuit gain increased through a new technique that neutralizes the detrimental effect of emitter inductance. This parameter has a serious effect on the high-frequency performance of transistor circuits. Equivalent circuits presently used to explain the operation of transistors can be very misleading and should be treated with utmost care. Many times one is led to believe that a circuit, such as a hybrid-pi, permits the accurate calculation of high-frequency performance. It is not generally recognized that all transistor parameters are not only functions of current but also of frequency, and that these circuits are at best a guide useful over a small frequency range.

Among several effects virtually neglected in presently accepted equivalent circuits are the internal feedback paths in transistors. For example, the base-emitter capacitance C'be contains a component that is due to internal Miller effect and exists even with the output shorted, because of intrinsic collector resistance and high transconductance, gm.

Also, as will be shown, an emitter inductance as small as 10 nh in conjunction with a C'be of 10 pf produces a resistive input component that reduces R'be, for example, from 1000 to 330 Ω at 100 mc. This effect follows at least a 6-db/octave curve and by itself could explain the loss of high-frequency power gain in transistors in a manner that is quite different from equivalent circuit concepts presently in vogue.

Similar effects are observed in tubes, but their magnitudes are much smaller because of the lower gm. In transistors, the intrinsic gm of the junction can be as high as 500,000 µmhos, or more, although the effective gm is much lower due to internal and external emitter impedances. An impedance of only 2 Ω, for example, reduces (g'm) by roughly 50%. Also, it easily can be calculated that with a load resistance of only 10 Ω, a voltage gain of 5 would be obtained through the use of g'm in the standard formula. Since the intrinsic collector resistance of a transistor is usually much higher than 10 Ω, the internal voltage gain and Miller effect—with output shorted—could be very high.

Considerations of this kind lead to very different equivalent circuits. R'be and C'be are no longer constants, but depend rather on the effectiveness of bypassing the emitter and can change drastically. The same is true for the parallel output resistance and capacitance of the transistors.

The total emitter inductance includes primarily (1) the inductance of the bonding wire from the emitter to the header pin, (2) the inductance of the transistor's emitter lead, and (3) the inductance of external leads and parts associated with the emitter circuit.

While certain improvements can be made to decrease the internal emitter inductance by careful design of the transistor case and header, the advantage gained is small due to the larger inductance normally inherent in external circuitry.

Total lead lengths, however, including the emitter lead, as well as those of associated circuit parts between the header of the transistor and input ground, do become important considerations. (Electronic Design, Dec. 6, 1961, p. 36)

See associated figure.

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