Chuck D. Bones
Circuit Wizard
What's a buffer?
In the simplest terms, a buffer is a circuit that amplifies the current, but leaves the voltage unchanged. The perfect buffer would have a voltage gain of 1.000, flat freq response, infinite input impedance and zero output impedance. Such a thing doesn't exist, but we can get close enough.
Why would we want one?
A buffer is desirable when we want to prevent one circuit from loading down another one. A simple example is a long cable. Cables have capacitance and the longer the cable, the more capacitance. A shielded cable can have a capacitance around 25pF per foot. A 10ft cable would have 250pF capacitance, which is not a big deal. A 100ft cable would be 2.5nF and that will affect the brightness of a single coil pickup. A buffer at or near the guitar will prevent that long cable from affecting the tone.
Another example: The Parenthesis has two buffers, one at the input and one at the output of the Rat circuit. Here's the input buffer. It gets its input from the guitar and drives the octave up and octave blend circuits. The octave up & blend pot would load the guitar down and the guitar's high output impedance would interfere with the operation of the octave blend control. This is a very simple buffer, only three parts. There is an identical buffer after the Rat filter which prevents the Booster or the next pedal in the chain from loading the filter circuit.
We can also build a buffer with a regular BJT. Here's the input buffer from the LGSM. Pretty much the same as the JFET buffer, above. The only difference is we have to bias the transistor from Vref instead of GND. Some JFET buffers use Vref to bias the gate.
We can also make a buffer with a MOSFET. This one is from the Pussy Melter. Very similar to the BJT buffer above.
Finally, we can use an opamp to make a buffer. This is from the Diezel VH4 (Valhalla). Note that the opamp is biased from GND because the opamp is powered by ±9V.
Which one is better? How do we decide which buffer circuit to use? In most cases, it comes down to the designer's personal preference. Within limits, they all perform the same. The opamp has the lowest output impedance and is capable of driving the most current. Whether you need that capability depends on what comes after it. If you have a spare opamp on the board, then it's a no-brainer.
Two of the circuit examples above have buffers and don't even need them. The opamp stage following the buffer in the LGSM and Pussy Melter have a high input impedance, so putting a buffer there doesn't help. It doesn't hurt, but it's not needed. In the LGSM, the buffer is a leftover from the Tube Screamer's soft switching. It's anyone's guess why Steel Panther put a MOSFET buffer in front of a MOSFET opamp.
How do they work?
Let's look at the JFET buffer above. It's wired as a source follower. When we raise the the voltage on the gate, the drain current increases, causing the source voltage to increase. The source voltage follows the gate voltage, hence the name "source follower." The nominal voltage gain is not exactly 1.00, but it's close enough. The input impedance of a JFET is over a gigaohm. R3 sets the input impedance of the circuit at 1M. The output impedance is 1 / gm where gm is the transconductance. Gm varies from one JFET to another and also depends on drain current. I measured a bunch of JFETs and their gm varied from about 0.5mS to 2mS at the kind of drain currents we run in pedals. That gives us an output impedance between 500Ω and 2K. We can go lower by dialing up the drain current, but this is low enough for most pedal applications.
The BJT & MOSFET buffers work exactly the same way. The output impedance is 1 / gm, just like the JFET. The gm of a BJT is given by gm = Ic / 26mV. If we run the BJT at 0.5mA, then gm = 19mS and the output impedance is 52Ω. Much lower than the JFET. A BJT's input impedance is also lower than a JFET, but one of the advantages of an emitter follower is the transistor's input impedance is multiplied by the HFE. So we end up with an input impedance in the 2M range if we use a BJT with HFE = 200. The biasing resistor, R3, is in parallel with that 2M, so we end up with around 400K input impedance. That's high enough to have little or no affect on a guitar's tone.
MOSFETs have an extremely high input impedance, therefore the buffer's input impedance is determined by the biasing resistor (R2 in the Pussy Melter). A BS170 has a transconductance around 30mS at the 2mA drain current in the example circuit above. That translates to 33Ω. At the same collector current, the BJT buffer above would have an output impedance around 13Ω.
Which one is best?
Opamp voltage followers use feedback to produce an input impedance over a gigaohm and an output impedance of a few milliohms. Clearly, they are superior on paper, but in many cases it would be difficult to hear the difference between the various buffer circuits described above.
Bottom line, any of them will work. The JFET & opamp buffers offer the lowest noise (provided you use a low-noise opamp). The opamp buffer is a freebie if you have a spare opamp on the board, otherwise the BJT buffer is the least expensive. MOSFETs are noisier than the rest, so I would not use a MOSFET as an input buffer in a high-gain pedal. As an output buffer, it's fine.
In the simplest terms, a buffer is a circuit that amplifies the current, but leaves the voltage unchanged. The perfect buffer would have a voltage gain of 1.000, flat freq response, infinite input impedance and zero output impedance. Such a thing doesn't exist, but we can get close enough.
Why would we want one?
A buffer is desirable when we want to prevent one circuit from loading down another one. A simple example is a long cable. Cables have capacitance and the longer the cable, the more capacitance. A shielded cable can have a capacitance around 25pF per foot. A 10ft cable would have 250pF capacitance, which is not a big deal. A 100ft cable would be 2.5nF and that will affect the brightness of a single coil pickup. A buffer at or near the guitar will prevent that long cable from affecting the tone.
Another example: The Parenthesis has two buffers, one at the input and one at the output of the Rat circuit. Here's the input buffer. It gets its input from the guitar and drives the octave up and octave blend circuits. The octave up & blend pot would load the guitar down and the guitar's high output impedance would interfere with the operation of the octave blend control. This is a very simple buffer, only three parts. There is an identical buffer after the Rat filter which prevents the Booster or the next pedal in the chain from loading the filter circuit.
We can also build a buffer with a regular BJT. Here's the input buffer from the LGSM. Pretty much the same as the JFET buffer, above. The only difference is we have to bias the transistor from Vref instead of GND. Some JFET buffers use Vref to bias the gate.
We can also make a buffer with a MOSFET. This one is from the Pussy Melter. Very similar to the BJT buffer above.
Finally, we can use an opamp to make a buffer. This is from the Diezel VH4 (Valhalla). Note that the opamp is biased from GND because the opamp is powered by ±9V.
Which one is better? How do we decide which buffer circuit to use? In most cases, it comes down to the designer's personal preference. Within limits, they all perform the same. The opamp has the lowest output impedance and is capable of driving the most current. Whether you need that capability depends on what comes after it. If you have a spare opamp on the board, then it's a no-brainer.
Two of the circuit examples above have buffers and don't even need them. The opamp stage following the buffer in the LGSM and Pussy Melter have a high input impedance, so putting a buffer there doesn't help. It doesn't hurt, but it's not needed. In the LGSM, the buffer is a leftover from the Tube Screamer's soft switching. It's anyone's guess why Steel Panther put a MOSFET buffer in front of a MOSFET opamp.
How do they work?
Let's look at the JFET buffer above. It's wired as a source follower. When we raise the the voltage on the gate, the drain current increases, causing the source voltage to increase. The source voltage follows the gate voltage, hence the name "source follower." The nominal voltage gain is not exactly 1.00, but it's close enough. The input impedance of a JFET is over a gigaohm. R3 sets the input impedance of the circuit at 1M. The output impedance is 1 / gm where gm is the transconductance. Gm varies from one JFET to another and also depends on drain current. I measured a bunch of JFETs and their gm varied from about 0.5mS to 2mS at the kind of drain currents we run in pedals. That gives us an output impedance between 500Ω and 2K. We can go lower by dialing up the drain current, but this is low enough for most pedal applications.
The BJT & MOSFET buffers work exactly the same way. The output impedance is 1 / gm, just like the JFET. The gm of a BJT is given by gm = Ic / 26mV. If we run the BJT at 0.5mA, then gm = 19mS and the output impedance is 52Ω. Much lower than the JFET. A BJT's input impedance is also lower than a JFET, but one of the advantages of an emitter follower is the transistor's input impedance is multiplied by the HFE. So we end up with an input impedance in the 2M range if we use a BJT with HFE = 200. The biasing resistor, R3, is in parallel with that 2M, so we end up with around 400K input impedance. That's high enough to have little or no affect on a guitar's tone.
MOSFETs have an extremely high input impedance, therefore the buffer's input impedance is determined by the biasing resistor (R2 in the Pussy Melter). A BS170 has a transconductance around 30mS at the 2mA drain current in the example circuit above. That translates to 33Ω. At the same collector current, the BJT buffer above would have an output impedance around 13Ω.
Which one is best?
Opamp voltage followers use feedback to produce an input impedance over a gigaohm and an output impedance of a few milliohms. Clearly, they are superior on paper, but in many cases it would be difficult to hear the difference between the various buffer circuits described above.
Bottom line, any of them will work. The JFET & opamp buffers offer the lowest noise (provided you use a low-noise opamp). The opamp buffer is a freebie if you have a spare opamp on the board, otherwise the BJT buffer is the least expensive. MOSFETs are noisier than the rest, so I would not use a MOSFET as an input buffer in a high-gain pedal. As an output buffer, it's fine.