Muting Signals with Photo-MOSFETS

In guitar pedals, photo-MOSFETs are commonly used to mute signals coming out of effects circuits — either by disconnecting the circuit in an Optical True Bypass, or by temporarily grounding the output signal in a Relay True Bypass while switching.

What’s a photo-MOSFET?

Photo-MOSFETs (a.k.a photofets, optofets, optocouplers, photocouplers, photorelays, …) are highly reliable, long life, and fast switching solid-state relays. They contain an LED on the control side, and MOSFETs in push-pull configuration as well as a Photo Diode Array (PDA) on the detector side. Both sides are electrically isolated.

When the control LED lights up, the photodiodes receive its infrared light and drive the MOSFET gates, thus turning on the MOSFETs and allowing current to flow. The two MOSFETs are serially connected in reverse, so they conduct bi-directionally when turned on.

Optical True Bypass

One way to mute a wet signal is to have the photofet conduct only when the pedal is switched on. When switched off, the photofet (ideally) is no longer part of the circuit and the effect is bypassed. That’s the Optical True Bypass.

With the DPDT switch in the “on” position as shown below, BOUT is connected to OUT. The MOSFETs on the detector side are conducting as the DPDT also grounds the infrared LED on the control side. Thus IN is connected to BIN via the photofet.

With the DPDT switch in the “off” position, the MOSFETs no longer conduct as the control LED is lifted from ground. IN is no longer connected to BIN and the effect send is grounded to discharge any coupling capacitors.

Shunting Audio to Ground

Another way to mute with photofets is to shunt the audio signal to ground. That’s commonly done in a Relay True Bypass, to avoid mechanical noise while the relay is switching.

As the control LED lights up, the MOSFETs conduct and shunt the signal to ground. That’s pretty much it.

Choosing Photo-MOSFETs

There are a few characteristics to consider when choosing photofets for either application. The most important ones are RON, the on-state resistance, COFF, the output capacitance, IFT, the trigger LED current, and VF, the forward voltage.

On-State Resistance R_ON

For an optical bypass that connects the input jack with the effect send via the photofet, RON isn’t actually that relevant. The on-state resistance effectively acts as a resistor in series at the input of the effects circuit and slightly increases its input impedance.

For common photofets such as the TLP222G (50 Ω), TLP222A (2 Ω), or H11F1M (200 Ω), RON is small enough that this effect is negligible. Even a low input impedance effect like the Fuzz Face wouldn’t change audibly.

R_ON in a Relay True Bypass

The on-state resistance is very relevant however when using photofets to shunt a signal to ground. For optimal muting it’s important to choose a photofet with the lowest possible RON, as it forms a voltage divider with the output impedance ZOUT of the effects circuit.

A RAT pedal, for example, has a J-FET buffer and a 100 kΩ pot as its output stage. The J-FET has a low output impedance (a few hundred ohms) and the pot adds up to 25 kΩ when dialed halfway. Let’s say we turn the volume knob almost all the way up, thereby reducing the output impedance.

Pick an H11F1M with RON = 200 Ω and the RAT with ZOUT = 1 kΩ. The photofet mute switch then gives an attenuation of only 1:6 or -15 dB – which isn’t much. A TLP222A with RON = 2 Ω yields a 1:500 or -53 dB attenuation, which is much better.

If all you got is a H11F1M and you want to make it work well with different circuits, then you can increase ZOUT by adding a series resistor between 1 kΩ and 10 kΩ before the photofet to achieve at least 20 dB attenuation.

Ultimately, the right value depends on the unknown load that is plugged into the output jack of the effects pedal. If the next stage has high input impedance, then even 10 kΩ to 47 kΩ might be reasonable as a series resistor.

Output Capacitance C_OFF

For both applications it’s important to choose a photofet with COFF as low as possible to minimize filtering of the audio signal.

COFF is the capacitance between the MOSFET pins when no current is applied to the control LED. The photofet then is effectively a capacitor to ground and forms a low-pass filter (LPF) with the output impedance ZPREV of the previous stage (for the optical bypass) or the output impedance ZOUT of the effects circuit (for the relay bypass).

The H11F1M, TLP222G and TLP222A have an off-state capacitance of 15 pF, 30 pF and 130 pF, respectively. 130 pF becomes problematic with an output impedance over 100 kΩ, as that starts to shave off guitar-band frequencies.

For example, with a 500 kΩ pot at the output, ignoring whatever comes before for simplicity, the maximum output impedance would be Zout = 125 kΩ. That gives a cut-off frequency of 9.79 kHz for the low-pass filter.

In the same scenario, a photofet with a lower output capacitance of, let’s say, 30 pF yields a much higher minimum cutoff frequency of about 42.44 kHz. Equally effective is ensuring a low output impedance via a buffer stage right at the end of the effects circuit (although that might affect attenuation as explained in the previous section).

Trigger LED Current I_FT and Forward Voltage V_F

It’s important that the LED lights up fully to either properly mute the signal or properly pass it. The control LED’s current limiting resistor should be chosen such that at least the maximum trigger LED current IFT is drawn.

If the datasheet says IFT = 1.6 mA typically, but 3 mA maximally, then pick the maximum value as the minimum supplied current. The datasheet guarantees that the LED is triggered with a least 3 mA. You can get away with typical values only by buying photofets in bulk and measuring IFT for each single one.

Use the maximum forward voltage VF to compute the required resistance of the current-limiting resistor RCLR. With V = 9 V and VF(max) = 1.3 V, you get RCLR = 7.7 V / 3 mA = 2.5 kΩ.

The minimum forward voltage VF(min) = 1 V then yields 3.2 mA, which is indeed larger than the minimum required trigger current.

Don’t forget to take resistor tolerance and other factors into account. If the power source is a battery, then V = 9 V might over time drop to, let’s say, 7 V. With RCLR = 2.5 kΩ, the available current would decrease to 2.28 mA and the photofet might no longer (fully) conduct.