LF356 — Monolithic JFET-Input Precision High-Speed Operational Amplifier

Overview

The LF356 (LF356N / LF356M) is an iconic monolithic JFET-input operational amplifier developed by National Semiconductor (now Texas Instruments). As the wide-bandwidth, fast-settling flagship of the pioneering BI-FET™ technology series, it integrates high-voltage JFETs on the same die with standard bipolar transistors to achieve low input bias currents ( $30\text{ pA}$ typical) without sacrificing high-frequency response.

With a $5.0\text{ MHz}$ gain bandwidth product, a slew rate of $12.0\text{ V/}\mu\text{s}$ , and an ultra-fast settling time of $1.5\ \mu\text{s}$ to $0.01\%$ , the LF356 has maintained a loyal following for decades in modular synthesizer circuits (analog VCOs, VCFs, and envelope generators), precision sample-and-hold amplifiers, logarithmic amplifiers, high-impedance electrometers, and DAC current-to-voltage converters.

Quick reference


Amplifier Type	Monolithic BI-FET™ JFET-Input Operational Amplifier
Package	8-pin PDIP (LF356N) / 8-pin SOIC (LF356M) / TO-99 Metal Can
Gain Bandwidth Product	$5.0\text{ MHz}$ typical
Slew Rate	$12.0\text{ V/}\mu\text{s}$ typical ( $7.5\text{ V/}\mu\text{s}$ minimum)
Settling Time (to 0.01%)	$1.5\ \mu\text{s}$ ( $10\text{V}$ step)
Input Bias Current ( $I_B$ )	$30\text{ pA}$ typical / $200\text{ pA}$ maximum ( $T_A = 25^\circ\text{C}$ )
Input Voltage Noise ( $e_n$ )	$12\text{ nV}/\sqrt{\text{Hz}}$ at $1\text{ kHz}$
Supply Voltage Range	$\pm 5.0\text{ V}$ to $\pm 18.0\text{ V}$ split (or $+10\text{V}$ to $+36\text{V}$ single)
Supply Current	$5.0\text{ mA}$ typical / $10.0\text{ mA}$ maximum

Pinout (DIP-8 / SOIC-8 Package)

text

        ┌──────────────┐
  BAL1  ─│ 1          8 │─ NC
   IN-  ─│ 2   LF     7 │─ V+
   IN+  ─│ 3   356    6 │─ OUT
    V-  ─│ 4          5 │─ BAL2
        └──────────────┘

Pin	Name	Description
1	`BAL1`	Offset balance terminal 1 (Connect $25\text{ k}\Omega$ trim pot wiper to V-)
2	`IN-`	Inverting analog input (JFET high-impedance gate)
3	`IN+`	Non-inverting analog input (JFET high-impedance gate)
4	`V-`	Negative power supply rail ( $-5\text{V}$ to $-18\text{V}$ or GND)
5	`BAL2`	Offset balance terminal 2
6	`OUT`	Analog operational amplifier output
7	`V+`	Positive power supply rail ( $+5\text{V}$ to $+18\text{V}$ )
8	`NC`	No internal connection

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Slew Rate	$SR$	7.5	12.0	—	$\text{V/}\mu\text{s}$	$V_S = \pm 15\text{V}, A_V = 1$
Small-Signal Bandwidth	$GBW$	—	5.0	—	MHz	$V_S = \pm 15\text{V}, T_A = 25^\circ\text{C}$
Settling Time to 0.01%	$t_s$	—	1.5	—	$\mu\text{s}$	$10\text{V}$ step, $A_V = -1$
Input Offset Voltage	$V_{OS}$	—	3.0	10.0	mV	$R_S \le 10\text{ k}\Omega$
Input Bias Current	$I_B$	—	30	200	pA	$T_A = 25^\circ\text{C}$
Equivalent Input Noise	$e_n$	—	12	15	$\text{nV}/\sqrt{\text{Hz}}$	$f = 1\text{ kHz}$
Common-Mode Rejection Ratio	$CMRR$	80	100	—	dB	$V_{CM} = \pm 10\text{V}$
Supply Current	$I_{CC}$	—	5.0	10.0	mA	$V_S = \pm 15\text{V}$

Common mistakes

JFET Input Phase Reversal: When input voltages approach the negative supply rail ( $V_-$ ), internal JFET drains can become forward-biased, inverting the output phase. In analog synth integrators and buffer circuits, clamp inputs with Schottky diodes or ensure signal swings remain within $\pm 11\text{V}$ on $\pm 15\text{V}$ rails.
Overlooking power dissipation: Drawing $5\text{ mA} \dots 8\text{ mA}$ quiescent current on $\pm 15\text{V}$ supplies means the package dissipates $\approx 200\text{ mW}$ statically, causing the package to feel noticeably warm to the touch. This self-heating increases input bias current slightly.
Grounding the offset balance pins: Pins 1 and 5 are internal balance nodes. Grounding them unbalances the internal differential stage. Leave unconnected unless actively trimming offset with a $25\text{ k}\Omega$ potentiometer tied to Pin 4 ( $V_-$ ).

Notes

BI-FET Series Family:
- LF355: Low supply current ( $2.0\text{ mA}$ ), $2.5\text{ MHz}$ bandwidth.
- LF356: Wide bandwidth ( $5.0\text{ MHz}$ ), fast settling ( $1.5\ \mu\text{s}$ ), $5.0\text{ mA}$ supply current.
- LF357: Decompensated high-gain wideband version ( $20\text{ MHz}$ , $50\text{ V/}\mu\text{s}$ , stable for $A_V \ge 5$ ).

Interface	Single Op-Amp Terminals with Offset Null
Package	8-pin PDIP / 8-pin SOIC / TO-99 Metal Can