LearnTronics

OPA2134PA

The OPA2134PA is the 8-pin PDIP version of Texas Instruments' OPA2134 dual FET-input audio operational amplifier. The FET input stage is especially useful with high-impedance sources such as piezoelectric pickups, where input current and loading can directly affect the signal.

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At a glance

Item	OPA2134PA reference value / note
Amplifiers per package	2 independent op amps
Package	8-pin PDIP (P), through-hole
Input stage	FET input
Input bias current	5 pA typical
Input voltage noise	8 nV/√Hz
Bandwidth	8 MHz
Slew rate	20 V/µs
Supply range	±2.5 V to ±18 V
Stability	Unity-gain stable
Recommended local bypassing	10 nF ceramic or larger at the supply pins

Reference: Texas Instruments OPAx134 data sheet, revised November 2024.

8-pin dual-op-amp pinout

Pin	Function
1	Output A
2	Inverting input A
3	Non-inverting input A
4	V−
5	Non-inverting input B
6	Inverting input B
7	Output B
8	V+

Why a FET input helps a piezo pickup

A piezo pickup is a high-impedance, largely capacitive source. If the following amplifier presents too small a resistance, the source is loaded and low-frequency output is reduced. The FJD guitar therefore uses a 10 MΩ input resistance on each string channel.

f_c = 1 / (2πRC)

The piezo capacitance C and the 10 MΩ resistance R form an approximate high-pass corner. The exact pickup capacitance must be measured or obtained from its data sheet before treating any result as the actual guitar response.

Illustrative piezo capacitance	With 10 MΩ, approximate fc
1 nF	15.9 Hz
2 nF	8.0 Hz
5 nF	3.2 Hz

These are examples showing the equation, not measured values for the actual FJD piezo elements.

Input bias current and a 10 MΩ source

Input bias current flowing through a source resistance can create a small DC error:

V = I × R

Using the OPA2134's 5 pA typical input bias current and 10 MΩ:

5 pA × 10 MΩ = 50 µV

That calculation demonstrates why a FET-input device is attractive for very high source resistances. A bipolar op amp with much greater input bias current could develop a much larger error across the same 10 MΩ resistance.

Supply and bypassing in the FJD guitar

The main guitar preamplifier uses two 9 V batteries as a split supply: +9 V, 0 V and −9 V. The OPA2134's ±9 V operating point lies comfortably within its specified ±2.5 V to ±18 V supply range.

Each physical OPA2134 package on the FJD PCB has local 10 nF bypass capacitors:

Pin 8 (+9 V) → 10 nF → ground
Pin 4 (−9 V) → 10 nF → ground

Texas Instruments recommends 10 nF ceramic capacitors or larger at the supply pins. The purpose is not audio tone shaping; the capacitors provide a short, local path for high-frequency supply current and help prevent supply wiring from becoming part of the amplifier's feedback problem.

Two amplifiers in one package

One OPA2134PA contains two independent op-amp channels. Three packages can therefore provide the six string buffers. The FJD circuit uses four physical OPA2134 packages in total for its complete analog path.

An unused op-amp section should not be left with floating inputs. A common safe arrangement is a voltage follower: connect the non-inverting input to a valid reference, then connect the output directly to the inverting input.

Used in LearnTronics

The OPA2134PA is central to the FJD Six-String Piezo Guitar Electronics project, where the six piezo strings are buffered before their individual 10 kΩ level controls and 22 kΩ summing resistors.

Texas Instruments data sheet

LearnTronics treats manufacturer data sheets as the authority for limits and specifications. Verify the exact manufacturer, suffix, package and revision before substituting or building.