What's a buffer, how do they work and why would we want one?
- Thread starter Chuck D. Bones
- Start date
spi
Well-known member
In a similar vein, I've wondered what goes into choosing the right value for this resistor?
Chuck D. Bones
Circuit Wizard
Or posted a meme.Uh oh! Shouldn’t have quoted that whole post…
steviejr92
Authorized Vendor
Sorry taking it down! Not the place for it!
Chuck D. Bones
Circuit Wizard
spi
Well-known member
yes, resistor between a transistor and vref/gnd
Chuck D. Bones
Circuit Wizard
There are two considerations:
1. How much DC current do we need to bias the transistor?
2. What load do we want to present to the previous stage (or device)?
The base current flows thru the bias resistor and we don't want too much voltage drop.
Often, that bias resistor is part of a high-pass filter. We choose the R & C to pass the right amount of bass.
An example is in order. Let's look at the LGSM TS clone input buffer. Vref is 4.5V and R3 is the bias resistor. Q1's collector current will be around 400μA. According to the datasheet, median HFE at Ic = 400μA is around 125. That give us a base current of (let's not see all the same hands now...)
Ib = 3.2μA.
That means we drop 1.6V in R3. That voltage drop eats some of our headroom, but we have plenty of headroom on the input side because a guitar signal might peak out a 1Vp-p if you have hot pickups and hit the strings real hard. 510K is a high enough resistance that we won't load the pickup appreciably.
Now let's look at the output buffer. I included C7, C8, R11 & the VOLUME pot because I want to suggest an improvement. R12, R13 & Q2 are the same as the input buffer, so we drop 1.6V in R12. We will have the same or more headroom if we set the VOLUME for unity. But if we dime the VOLUME, we'll have less headroom. Probably not a big deal if we keep TONE below noon because the max output from the 2nd stage will be on the order of 2Vp-p. But... There is no problem with loading the volume pot a little bit, so we could reduce R12 to 100K which in turn reduces the voltage drop to 0.32V and increases the headroom. We could also use a higher HFE transistor for Q2, which would also reduce the voltage drop in R12. We could go a step further. The output of IC1.2 is biased to Vref. The VOLUME control is biased to Vref and so is Q2's base. So why do we need C7, C8 & R12? We don't. We can remove all three parts and jumper C7 & C8. Now the maximum resistance between Q2's base & Vref is 100K. As we turn VOLUME down, the resistance gets lower. Why are C7, C8 and R12 even in there? Because they are leftovers from the Tube Screamer's soft switching. There might be some slight scratching when VOLUME is turned since Q2's bias will change a little bit when VOLUME is turned. Probably not audible and even if it is, so what? How often do we turn the VOLUME knob?
1. How much DC current do we need to bias the transistor?
2. What load do we want to present to the previous stage (or device)?
The base current flows thru the bias resistor and we don't want too much voltage drop.
Often, that bias resistor is part of a high-pass filter. We choose the R & C to pass the right amount of bass.
An example is in order. Let's look at the LGSM TS clone input buffer. Vref is 4.5V and R3 is the bias resistor. Q1's collector current will be around 400μA. According to the datasheet, median HFE at Ic = 400μA is around 125. That give us a base current of (let's not see all the same hands now...)
Ib = 3.2μA.
That means we drop 1.6V in R3. That voltage drop eats some of our headroom, but we have plenty of headroom on the input side because a guitar signal might peak out a 1Vp-p if you have hot pickups and hit the strings real hard. 510K is a high enough resistance that we won't load the pickup appreciably.
Now let's look at the output buffer. I included C7, C8, R11 & the VOLUME pot because I want to suggest an improvement. R12, R13 & Q2 are the same as the input buffer, so we drop 1.6V in R12. We will have the same or more headroom if we set the VOLUME for unity. But if we dime the VOLUME, we'll have less headroom. Probably not a big deal if we keep TONE below noon because the max output from the 2nd stage will be on the order of 2Vp-p. But... There is no problem with loading the volume pot a little bit, so we could reduce R12 to 100K which in turn reduces the voltage drop to 0.32V and increases the headroom. We could also use a higher HFE transistor for Q2, which would also reduce the voltage drop in R12. We could go a step further. The output of IC1.2 is biased to Vref. The VOLUME control is biased to Vref and so is Q2's base. So why do we need C7, C8 & R12? We don't. We can remove all three parts and jumper C7 & C8. Now the maximum resistance between Q2's base & Vref is 100K. As we turn VOLUME down, the resistance gets lower. Why are C7, C8 and R12 even in there? Because they are leftovers from the Tube Screamer's soft switching. There might be some slight scratching when VOLUME is turned since Q2's bias will change a little bit when VOLUME is turned. Probably not audible and even if it is, so what? How often do we turn the VOLUME knob?
jesuscrisp
Well-known member
Removing those coupling caps plus the one between input buffer and gain stage are among my favorite TS mods!
Overall great thread on buffers, only thing I slightly disagree with is the Rat example. The JFET buffer sits after the tone stage and before the volume pot, where most likely it's job was to seperate the two so the volume knob wouldn't affect tone. However, there's still the volume knob after the JFET which would cause the output impedance of the circuit to vary depending on where the pot is set. Thus it's not doing the typical job of an output buffer, which would be to provide consistent low output impedance.
Overall great thread on buffers, only thing I slightly disagree with is the Rat example. The JFET buffer sits after the tone stage and before the volume pot, where most likely it's job was to seperate the two so the volume knob wouldn't affect tone. However, there's still the volume knob after the JFET which would cause the output impedance of the circuit to vary depending on where the pot is set. Thus it's not doing the typical job of an output buffer, which would be to provide consistent low output impedance.
Chuck D. Bones
Circuit Wizard
I guess that depends on your definition of "low." The Rat VOLUME control could be reduced to 10K, then the output impedance would vary from 0Ω to about 2.7K. In any case it's a buffer and it's at the output end of the circuit.
