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Collector Drive Circuit With High Input Impedance

IP.com Disclosure Number: IPCOM000085454D
Original Publication Date: 1976-Apr-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 2 page(s) / 46K

Publishing Venue

IBM

Related People

Gruodis, AJ: AUTHOR

Abstract

The basic circuit depicted in Fig. 1, meets the requirements of Equations 1, 2 and 3. The circuit operates as follows:.

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Collector Drive Circuit With High Input Impedance

The basic circuit depicted in Fig. 1, meets the requirements of Equations 1, 2 and 3. The circuit operates as follows:.

If the input is down, the current I1 flows completely through the emitter of transistor T1 and Equation 3 guarantees that transistor T2 is turned off.

If the input is up, then the input current is of such magnitude as to supply current in resistor R2 and the base current for transistor T2 turning it on. I1 supplies the base current for transistor T1. The output will drop until diode D3 turns on and reduces the base drive for transistor T1 to a point where the collector current (T1 and input current through diode D1) is just sufficient to keep T2 on. Raising the input to a higher potential does not increase the current, producing an apparent high-input impedance.

The basic circuit can be modified to perform different functions.

Example 1. To convert this to an off-chip driver, with controlled rise time and low downlevel, diode D3 is replaced with a pair of series diodes (SBD or PN) and a shunt capacitor. Note that the capacitor can be made relatively small, since current I1 can be made approximately equal to I LOAD MAX/Beta/2/. It should also be noted that the power dissipation of the input network is proportional to the existing load and not the worst-case design. This allows the use of the same driver design over a large load range without excessive power penalty when driving small loads.

Exampl...