Table 9-1. Composite Amplifier Component List and Function

COMPONENT

DESCRIPTION

RELATED EQUATION

CB

DC blocking capacitor for input source. Use a dc blocking capacitor

if the dc voltage of the input source is not the same as the gate bias

voltage.

 f–3dBDC=  1

2* π*RB1 RB2*CB1

(2)

RB

Bias resistor. Use biasing resistors to set the dc voltage at the

gate. High-value resistors can be used without an impact to noise

if the source impedance and bypass capacitor have sufficiently low

impedance.

See Equation 2

RG

Gate resistor. Can be used to help limit current flow into gate in

overvoltage cases.

RD

Drain resistor. Sets gain of JFET stage in common source biasing,

along with gm and RS.

RS

Source resistor. Used to set bias of JFET; see Figure 9-3. Resistor

thermal noise directly impacts noise performance.

CD

DC blocking capacitor. Blocks nominal drain voltage so the amplifier

operates at a midsupply bias point.

CF

Feedback capacitor. Along with RF, this capacitor sets the –3‑dB

high-pass cutoff frequency when the amplifier gain-bandwidth product

(GBW) is sufficiently high enough to support the –3

‑dB frequency.

If the GBW is not high enough, then the GBW sets the –3-dB

frequency.

 f–3dBHP=  12*π*RF*CF

(3)

RF

Feedback resistor. Along with CF, this resistor sets the –3‑dB high-

pass cutoff frequency when the amplifier gain-bandwidth product

(GBW) is sufficiently high enough to support the –3

‑dB frequency.

If the GBW is not high enough, then the GBW sets the –3-dB

frequency.

See Equation 3

RF2

Current feedback gain-setting resistor 1. Along with RS2, sets gain

closed-loop.

 VOUTVIN= RF2RS2

(4)

RS2

Current feedback gain-setting resistor 2. Along with RS2, sets

gain closed-loop. Resistor thermal noise directly impacts noise

performance.

See Equation 4

CS

Current feedback ac-coupling capacitor. This capacitor, along with R2,

sets the low-pass –3-dB frequency.

 f–3dBLP=  12*π*RS2*CS

(5)

9.2 Typical Application

The JFE150 can be configured to provide a low-noise, high-input-impedance front-end stage for a typical op

amp. Single-transistor gain stages shown previously suffer from wide gain variations dependent on the forward

transcondutance of the JFE150. When precision gain is required, the composite amplifier (JFET front-end +

operational amplifier) achieves excellent results.

12 V

JFE 150

VIN

RD

RS

+

±

+

±

+

±

CS

F

CS

F

10 k

10 k

+

±

+

±

+

±

RG

RG

RS2

RS2

OPA 202

VOUT

CF

10 pF

CF

10 pF

CD

F

CD

F

6 V

RB

1 M

RB

1 M

CB

F

CB

F

Figure 9-6. Low-Noise, High-Input-Impedance Composite Amplifier

JFE150

SLPS732B – JUNE 2021 – REVISED APRIL 2023

www.ti.com

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