On-chip series terminated CMOS(Bi-CMOS) driver

A terminated driver circuit, comprising:

• an external impedance;
• a bias generator circuit connected to the external impedance, said bias generator circuit generating a plurality of bias voltages in response to a reference current, said reference current being a function of the external impedance magnitude, wherein said bias generator circuit further comprises:
• a reference current circuit connected to the external impedance, wherein the reference current circuit compares an internal reference voltage to a voltage which exists across the external impedance and provides feedback to the external impedance in response to the comparison by adjusting a current sourced through the external impedance and thereby stabilizing the current magnitude of the current sourced through the external impedance when the internal reference voltage is equal to the voltage across the external impedance, wherein a reference current is created that is a ratio of the current magnitude sourced through the external impedance;
• and
• a bias voltage reference circuit connected to the reference current circuit, wherein the bias voltage reference circuit takes said reference current from said reference current circuit and sources said reference current through a transistor thereby creating a second voltage which is compared to a second reference voltage, and wherein feedback is supplied as a voltage to a control terminal of said transistor thereby adjusting the second voltage until said second voltage is equal to said second reference voltage, whereby the voltage at said control terminal of the transistor is said bias voltage for said bias voltage reference circuit;
• and
• an output driver circuit connected to said bias generator circuit, said output driver circuit having a plurality of output devices connected to a transmission line, said driver circuit receiving said plurality of bias voltages from said bias generator circuit and selectively coupling them to the plurality of output devices such that only a single bias voltage is driving a single output device at a time, wherein said plurality of bias voltages causes the plurality of output devices to have a specific output impedance that is matched with the characteristic impedance of said transmission line thereby reducing waveform reflections.

A method of controlling a circuit output impedance, comprising the steps of:

• establishing a reference current through a known impedance;
• generating a fixed voltage in response to said reference current;
• impressing a voltage substantially equal to said fixed voltage across a transistor having substantially similar operating characteristics as an output device;
• biasing said transistor to conduct a ratio of said reference current, thereby creating a bias voltage;
• and
• transmitting the bias voltage to a control terminal of the output device, whereby said output device, when driving a load, exhibits a controlled output impedance.