My appearance in 1996Wayne Stegall

Copyright 2010

Created January 2, 2010
Updated January 27, 2010.  See document history at end for details.

Active Inductor Example

JFET Phono Preamp

This is an example only.  It is not intended to be final or complete design.

Figure 1:  Schematic

Parts List

Q1-Q4 2N3918 n-ch jfet
Q5,Q6 BS170 n-ch mosfet
220Ω, 5%
220Ω, 5%
165kΩ, 1%
23.7kΩ, 1%
13nF, 1% or 5%
240pF, 1% or 5%
C2N,a 4.3nF, 1% or 5%
240pF, 1% or 5%
R1L 105kΩ, 1%
CL 10,000F, 25V electrolytic
1kΩ, 5%
82Ω, 5%
1kΩ, 5%
100F, 25V electrolytic
1kΩ pot
S1,S2 SPST reed relay, normally closed, coil rated 10mA@5V

Remaining values are left for you to calculate.  See text.

Initial Design Decisions

vg,Q5 = (VDD-vgs-Q5-max)+vsg-Q1-max
= 12V

Gain Stage Design

Although I used a convenient fourier series analysis tool to do the small signal analysis, there were some design decisions.  I wanted to choose gate and source resistors small enough to make their noise contribution insignificant compared to the JFET.  This would lower noise and allow the possiblity of noise improvement with better JFETs.
Resistor noise equation (per rtHz, figure of merit, does not account for bandwidth)
vn-resistor-rtHz = sqrt(4kTR)
Rearrange resistor noise equation to calculate equivalent input noise resistance of JFET.
R =
4 x 1.3806504e-23 x 298.15K
 = 2.186371576kΩ
I choose RG and RS 10 times lower to make their noise contribution insignificant.
R1-R8 = 220Ω.
Because the fourier series analysis resulting from this choice was excellent, I made no further adjustments to these values.

Fourier Series Analysis of Q1-Q4 (excluding drain load)

Analysis of Common Source Circuit
transistor type = JFET
transconductance = 5.5mS
threshold voltage = -3V
source resistor = 220Ω

Input signal (peak) = 7.0710678mV
Input bias = 0V
Output signal (peak) = 21.585572A
Output bias = 7.8662828mA1
Gain = 3.0526609mS

Total distortion = 1.6210607nA = 0.00750993% = -82.4873dB

Breakdown by harmonic

harmonic value percent dB
0 7.8662844mA1 36442.32534044% 51.23dB DC Offset
1 21.585572A 100.00000000% 0.00dB Fundamental
2 1.6210607nA 0.00750993% -82.49dB

RIAA Analysis

Refer Calculating Passive RIAA Equalization in article Phono Equalization Calculations.

From this fourier analysis we can calculate R1 of Passive RIAA network (aka R2L)
R2L =
(Low frequency RIAA boost relative to 1kHz) x (1kHz output voltage)
(Output signal current) x (#parallel devices)
10 x 1.414213562V
21.585572A x 4
= 163.791532kΩ
Round to nearest 1% value.  R2L = 165kΩ

Given R1:R2 = 6.877358491
R2N = 165kΩ
= 23.99176955kΩ
Round to nearest 1% value.  R2N = 23.7kΩ

Given R1C1 = 2187S
C1N =
= 13.25454545nF
Round down to nearest 5% value: 13nF
Choose parallel 5% value to makeup remainder:  240pF
C1N = 13nF + 240pF

Given R1C2 = 750S
C2N =
= 4.545454545nF
Round down to nearest 5% value: 4.3nF
Choose parallel 5% value to makeup remainder:  240pF
C2N = 4.3nF + 240pF

Active Inductor Analysis

Refer article Active Inductor Load.
R2L chosen already in RIAA analysis.
BS170 specification:  gm = 320mS.
gfs = 2 x sqrt(gmiD) = 2 x sqrt(320mS x 7.8662828mA x 4) = 200.6872391mS
R1L =


left-paren Vbias

left-paren 6V
2V+sqrt((7.8662828mA x 4)/320mS)
 = 103.5527796kΩ
Round to nearest 1% value:  R1L = 105kΩ
CL =
(200.6872391mS x 105kΩ)+1
2π x 5Hz x 105kΩ
 = 6.388376376mF
6800F would work here.  I would rather choose a larger common value (CL = 10,000F) and solve for a lower pole.
(200.6872391mS x 105kΩ)+1
2π x 10mF x 105kΩ
 = 3.194188188Hz
Power on relay added at beginning of design.
Calculate charge time for CL:
i = C δv

Rearrange capacitor equation above and calculate time.  (2V presumes vgs not much more than vT)
Δt =
10mF x 2V
7.8662828mA x 4
= 635.6242366mSec

Q6 Circuit Design

My design goal here was reasonably low output impedance.  1kΩ output impedance would have been low enough to minimize voltage loss into a typical 47kΩ load.  I choose 500Ω instead on the far chance of driving a 600Ω load with a tolerable voltage loss.

Fourier Series Analysis of Q6 Circuit

Note:  This analysis based on a more convenient equivalent model of source circuit.  Two 500Ω are modeled in series with one bypassed by a capacitor in place of the parallel circuit shown.
Analysis of Common Source Circuit
transistor type = MOSFET
transconductance = 320mS
threshold voltage = 2V
source resistor = 500Ω

Input signal (peak) = 1.4142136V
Input bias = 7V
Output signal (peak) = 2.7910912mA
Output bias = 9.8041739mA1
Gain = 1.9735995mS

Total distortion = 950.79116nA = 0.03406521% = -69.3538dB

Breakdown by harmonic

harmonic value percent dB
0 9.805058mA1 351.29836800% 10.91dB DC Offset
1 2.7910912mA 100.00000000% 0.00dB Fundamental
2 888.84698nA 0.03184586% -69.94dB
3 56.727241nA 0.00203244% -93.84dB
4 4.7600566nA 0.00017054% -115.36dB
5 456.88194pA 0.00001637% -135.72dB

Miscellaneous Calculations

R12 limits charge current through Q6 to C2 to <500mA
R12 >= 20V/500mA = 40Ω
Choose R12 = 82Ω.

Want to set C2 for as low a highpass pole as CL.  First find Requiv of pole.  Output impedance of Q6 is the inverse of its operating ac transconductance gfs.
gfs = 2 x sqrt(gmiD) therefore
Rout-Q6 =
2 x sqrt(gmiD)
2 x sqrt(320mS x 9.8041739mA)
 = 8.92667066Ω

Requiv = (Rout-Q6 || R13) + R15 = Rout-Q6 x R13
Rout-Q6 + R13
+ R15 = 1.00884769kΩ

C2 =
2π x 2.348779238Hz x 1.00884769kΩ
 = 67.16643973F
Choose C2 = 100F.

Choose R10 and  R14 to set 10mA through relay coil.
R10 =
 = 1.3kΩ

Calculate load current from power supplies VDD1 and VDD2.
iDD1 = iQ1-BIAS x 4 + iRELAYCOIL = 7.8662828mA x 4 + 10mA  = 41.4651312mA
iDD2 = iQ6-BIAS + iRELAYCOIL = 9.8041739mA + 10mA = 19.8041739mA

Design Decisions Left to You

C1 must be chosen by calculation or experimentation to exceed CL charge time calculated above (432.2244809mSec).  C3 would then be chosen for a somewhat longer delay to prevent a pop from the activation of the other relay.

1There is a seeming discrepancy in the fourier analysis between the cited output bias and the same value given in the analysis.  The first value is the bias without any signal and the second with a signal. This effect is created by the same asymetrical transfer curve that gives us a desireable second order distortion characteristic.  The positive half of the signal is amplified more than the negative half.  As a result the dc bias is modulated by the average signal level. This small component amounts to AM demodulation of the  music signal.  Abstractly, I had already anticipated this effect.  I would be curious what effect this has on the overall sound.  Is is detrimental, or does it add an extra euphonic bass urge?

Document History
January 2, 2010  Created
January 2, 2010  Minor Updates.
January 5, 2010  Recommend split power supply to prevent feedback.  Update schematic to show change.  Recommend specific power supply.
January 5, 2010  Choose relays S1 and S2, calculate their series resistors, and calculate specific load current for each power supply tap.
January 9, 2010  Added footnote explaining how fourier series analysis shows modulation of dc bias by music signal.
January 17, 2010  Added design criteria for class-a biasing decisions.  Recalculate Active Inductor Analysis due to miscalculation of gfs.
January 23, 2010  Correct schematic in vicinity of Q4, R7, and R2N.
January 27, 2010  Minor improvement to writing style.