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Diode Connected Transistor As Load

IP.com Disclosure Number: IPCOM000048755D
Original Publication Date: 1982-Mar-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Betts, R: AUTHOR

Abstract

A diode-connected transistor. when used as a load element, compensates for the fall-off in current rise time of the control transistor with increasing frequency.

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Diode Connected Transistor As Load

A diode-connected transistor. when used as a load element, compensates for the fall-off in current rise time of the control transistor with increasing frequency.

Fig. 1 shows a transistor Q1 having its base B connected to its collector C with an impedance Rb. This impedance may be only the normal base resistance, or it can include an external impedance.

When Q1 is used as a load in a practical circuit such as Fig. 2, the current from C to E will be determined by the collector current of Q2 as a current source. The rise time of this current is limited by the cutoff frequency of Q2. A typical transistor, whose cutoff frequency is 500 MHz, will have a collector current rise time of 10 to 15 nanoseconds in response to a step function voltage at its base. If the cutoff frequency of Q1 is less than that of Q2, the current from C to E in Q1 will divide; initially, because of the low current gain at high frequencies, more than the normal base current will flow through Rb and produce a larger voltage from C to E than would be expected at lower frequencies. Gradually, this portion of the current from C to E will decrease, and the proportion of voltage from C to E resulting from the current through Rb will also decrease.

The net effect is similar to the use of an inductance in series with a load resistor to increase the high frequency response of a transistor voltage amplifier.

The circuit of Fig. 2 uses three diode-connected transistors a...