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Bifet VCO With Improved Frequency Range

IP.com Disclosure Number: IPCOM000035349D
Original Publication Date: 1989-Jul-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Buchholtz, TC: AUTHOR [+3]

Abstract

An integrated general-purpose VCO for phase-locked loops is described herein. The frequency of the VCO is set by a control voltage that is converted to a current by external resistors. The frequency of the VCO is proportional to this current. The circuit is novel in that it uses the frequency controlling input current to improve the frequency range of operation. (Image Omitted)

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Bifet VCO With Improved Frequency Range

An integrated general-purpose VCO for phase-locked loops is described herein. The frequency of the VCO is set by a control voltage that is converted to a current by external resistors. The frequency of the VCO is proportional to this current. The circuit is novel in that it uses the frequency controlling input current to improve the frequency range of operation.

(Image Omitted)

Fig. 1 shows a schematic of the VCO cell. The controlling current of the VCO (Ivco) enters the AI0 pin of the circuit. Diode-connected NFET T1 mirrors Ivco in NFET current sources T2, T3, and T4. The on chip VCO timing capacitor is formed by C16 and C17. NFET current source T2 drives diode-connected PFET T7, which mirrors the current into PFETs T8 and T9, which operate at 4/3 * Ivco, as long as their drains are a few tenths of a volt below P5V. NPN transistors Q3 and Q4, along with resistor 1 and NFET T18, form a clamp so that nodes C and D cannot go more than about 1.3 volts below the power supply. NFETs T23 and T24 form a cross-coupled differential pair which cause the circuit to oscillate.

When the VCO is oscillating, NFETs T23 and T24 are switching at the frequency of oscillation. When T23 is off, T24 is on and Ivco flows through the capacitors C16 and C17 from right to left. During this time, T24 conducts 2*Ivco, PFET conducts 4/3*Ivco, and NPN Q3 conducts 2/3*Ivco clamping node C at P5V-1.3 volts. Meanwhile, node D is at P5V so PFET T8 and NPN...