LM13700 — Dual Operational Transconductance Amplifier (OTA)

Overview

The LM13700 (commonly LM13700N in a 16-pin DIP package) is a dual Operational Transconductance Amplifier (OTA) manufactured by Texas Instruments (originally National Semiconductor). Unlike standard op-amps whose output voltage is proportional to input differential voltage, an OTA converts differential input voltage into an output current ( $I_{OUT} = g_m \times V_{IN}$ ), where transconductance ( $g_m$ ) is directly controlled by an external Amplifier Bias Current ( $I_{ABC}$ ).

Integrated with linearizing input diodes (which extend linear signal dynamic range by $10\text{ dB}$ ) and independent Darlington output buffers, the LM13700 is the quintessential chip for DIY analog music synthesizers, powering Voltage-Controlled Amplifiers (VCAs), Voltage-Controlled Filters (VCFs), and Voltage-Controlled Oscillators (VCOs).

Quick reference


Device Type	Dual Operational Transconductance Amplifier (OTA)
Package	16-Pin DIP (Through-Hole) / SOIC-16
Supply Voltage Range ( $V_{CC}$ )	$\pm 4.75\text{ V}$ to $\pm 18.0\text{ V}$ ( $9.5\text{ V}$ to $36.0\text{ V}$ single supply)
Transconductance Range ( $g_m$ )	$0$ to $10,000\ \mu\text{S}$ ( $10\text{ mS}$ ) controlled by $I_{ABC}$
Max Bias Current ( $I_{ABC}$ )	$2.0\text{ mA}$ maximum per channel
Linearizing Diodes	Reduces input distortion; allows larger signal levels ( $\pm 50\text{mV}$ )
Integrated Buffers	Independent Darlington voltage followers for low-impedance output driving

Pinout (16-Pin DIP Package)

text

        ┌──────────────┐
  1I_ABC ─│ 1         16 │─ 2I_ABC
  1 D_B ─│ 2         15 │─ 2 D_B
   1+IN ─│ 3         14 │─ 2+IN
   1-IN ─│ 4         15 │─ 2-IN
  1 OUT ─│ 5         12 │─ 2 OUT
     V- ─│ 6         11 │─ V+
 1BUF_I ─│ 7         10 │─ 2BUF_I
 1BUF_O ─│ 8          9 │─ 2BUF_O
        └──────────────┘

Pin	Name	Description
1	`1 I_ABC`	Amp 1 Amplifier Bias Current input ( $I_{ABC1}$ )
2	`1 Diode Bias`	Amp 1 Linearizing diode bias current input
3	`1 +IN`	Amp 1 Non-inverting input pin
4	`1 -IN`	Amp 1 Inverting input pin
5	`1 OUT`	Amp 1 High-impedance current output pin
6	`V-`	Negative power supply rail ( $-15\text{V}$ for dual supply, $0\text{V}$ for single supply)
7	`1 Buffer IN`	Amp 1 Darlington buffer base input pin
8	`1 Buffer OUT`	Amp 1 Darlington buffer emitter output pin
9	`2 Buffer OUT`	Amp 2 Darlington buffer emitter output pin
10	`2 Buffer IN`	Amp 2 Darlington buffer base input pin
11	`V+`	Positive power supply rail ( $+15\text{V}$ for dual supply)
12	`2 OUT`	Amp 2 High-impedance current output pin
13	`2 -IN`	Amp 2 Inverting input pin
14	`2 +IN`	Amp 2 Non-inverting input pin
15	`2 Diode Bias`	Amp 2 Linearizing diode bias current input
16	`2 I_ABC`	Amp 2 Amplifier Bias Current input ( $I_{ABC2}$ )

Transconductance Formula

The open-loop transconductance ( $g_m$ ) of each OTA channel is directly proportional to its bias current ( $I_{ABC}$ ):

$g_m \approx 19.2 \times I_{ABC} \quad (\text{at } T_A = 25^\circ\text{C})$

When linearizing diodes are biased, transconductance is calculated as:

$g_m \approx \frac{I_{ABC}}{2 V_T}$

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Transconductance ( $I_{ABC} = 500\mu\text{A}$ )	$g_m$	6700	9600	13000	$\mu\text{S}$	$T_A = 25^\circ\text{C}$
Transconductance Tracking	$\Delta g_m$	—	0.4	1.0	dB	Channel-to-channel match
Input Offset Voltage	$V_{IO}$	—	0.4	4.0	mV	$I_{ABC} = 500\mu\text{A}$
Input Bias Current	$I_{IN}$	—	0.4	2.0	$\mu\text{A}$	$I_{ABC} = 500\mu\text{A}$
Peak Output Current	$I_{OUT}$	350	500	—	$\mu\text{A}$	$I_{ABC} = 500\mu\text{A}$

Common mistakes

Applying input signals larger than 20mV without linearizing diodes: Unbiased OTA inputs overdrive easily. Input signals above $\approx 20\text{ mV}$ peak without diode bias produce heavy harmonic clipping. Attenuate input signals or bias Pin 2/15 (Diode Bias).
Connecting high capacitive loads directly to output pin (Pin 5/12): The OTA output is a high-impedance current source. Connect Pin 5 to the internal Darlington buffer input (Pin 7) or an external op-amp current-to-voltage converter stage.

Notes

Synthesizer VCA Core: The LM13700 is the most popular OTA IC in DIY synth building (Erica Synths, MFOS, Doepfer modules), used in voltage-controlled gain cells, low-pass state-variable filters, and ring modulators.

Interface	Dual OTA Inputs (+IN, -IN, I_ABC), Linearizing Diode Bias, Dual Output Buffers
Package	16-pin PDIP / SOIC-16