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IEEE Indicon 2014, 11-13 Dec, 2014, Pune, India, Paper No. 1098 (Poster session: Thursday, 13th Dec, 0900-1300)
A JFET-based Circuit for Realizing a Precision and
Linear Floating Voltage-controlled Resistance
Rani Holani, P. C. Pandey, and Nitya Tiwari
Indian Institute of Technology Bombay
Abstract—A JFET-based circuit for realizing a precision and
Circuit description
linear floating voltage-controlled resistance (VCR) is presented
for use in analog multipliers and programmable analog
circuits and as a resistance mirror. It uses a matched JFET
pair along with an op amp based negative feedback for
realizing a precision resistance and a feedback of the source
and drain voltages to the gate for realizing a linear floating
resistance. The circuit operation is validated through circuit
simulation and practical testing.
Operation
Background
Voltage controlled-resistance (VCR) applications
◦ Modulators & demodulators
◦ Programmable analog circuits
◦ Analog multipliers
◦ Volume controllers
• Matched transistors Q1, Q2 in triode region
• Q1 channel resistance stabilized against parameter variations using
negative feedback (op amp A1)
• Linearity extended by source-drain bootstrapped gates
• Q2: Floating resistor with terminals X and Y
Equations
JFET (or MOSFET) based grounded VCR
+
RDS = vDS/iD = 1/[k(vGS − vP − vDS /2]
vDS << vGS −VP
D
vDS
+
G
vGS
• Linear only for tens of mV of vDS
• Imprecision due to temperaturedependent & piece-to-piece variations
• Not useable as floating VCR.
S
−
−
 i1 = − v1/R1
▪ vG1 = (vC + v2)/2
▪ vG2 = (vC + vX + vY)/2
 iX = (vX − vY)/[v2/(− v1/R1)], 0 ≥ vC ≥ 2VC & │vX − vY │≤ min(−vC , vC − VP)
 RXY = [v2/(− v1)] R1
Features
• Voltage controlled: by v1 or v2
• Current controlled: by current source i1 in place of v1 and R1
• Floating VCR: neither X nor Y needs to be grounded
• Resistance mirror: vC applied for controlling multiple X-Y terminal pairs
Simulation results
JFET-based floating VCR (Senani, 1994)
iX
R4
R5
+
−
M1
R3
A1
+
vC +
−
X
vXY
vG
−
iY
R1
R2
Y
Source-drain
bootstrapped gate: vD
& vS superimposed on
the control voltage
• Floating VCR with
an extended range
of linearity
• Imprecision due to
temperaturedependent & pieceto-piece variations
Precision grounded VCR (Clarke, 1977)
vREF
iX
+
−
D
S
R1
vC
+
−
i1
M1
+
v G M2
−
G
−
A1
+
iY
X
vXY
Y
Self tracking ckt
using matched JFET
pair & op amp based
negative feedback to
compensate for
variations in the
device parameters
• Not useable as a
floating VCR
• Useable only for
small terminal
voltages
Simulation using LTspice IV for examining the effect of variation in device
parameters (JFET pair U441, op amp LT1366 with ±15 V, v2 = 1.0 V, R1 = 1000 Ω, vY = 0,
variable vX)
A) Operation of A1-Q1 selftracking: Effect of variation in
VP and IDSS on vC, with v1 as
input
B) Operation of full ckt:
Effect of variation in vXY, VP
and IDSS on RXY
• Single device: large change with
vXY & device parameter variations
• Proposed ckt: < 5% change with
vXY, variation, no dependence on
device parameters
Experimental results
Ckt components: JFET pair U441, op amp LT1366 with ±15 V, R2 = R3 = 1 MΩ, R4 =
R5 = R6 = R7 = 12 kΩ, R8 = 6 kΩ (12 kΩ ║ 12 kΩ).
• Test condition: v1 = −1 V, v2 = 1 V, R1 = 1 kΩ, observed vC = −3.68 V, variable vX, vY = 0.
Result: RXY = 1033 Ω with −2.7% to 2.0% change for vXY of ± 1 V.
• Test condition: v1 = −0.5 V, v2 = 1 V, R1 = 1 kΩ, observed vC = −4.85 V, variable vX,
vY = 0.
Proposed circuit
A JFET-based circuit for precision & linear floating
VCR & resistance mirror, using the features of selftracking & source-drain bootstrapped gate circuits.
Result: RXY = 2039 Ω with −4.2% to 2.2% change for vXY of ± 1 V.
Conclusion
Precision & linear operation of the proposed floating VCR circuit
validated using simulation and practical testing.
Applications: voltage or current-controlled time-varying resistance, a
resistance mirror for controlling resistances across a set of ports.