Bootstrapping is a potentially powerful technique for boosting input impedance and gain of an amplifier. Illustrated here are the two basic uses of bootstrapping.
Great video, I tried your boostrapping gain boost but I replaced the input stage with a JFET circuit. Great tube sounding guitar boost-overdrive. Thank you!
As I understand it, it ultimately produces a bucking voltage for your input signal so it's seeing a much higher impedance when it appears across the new input resistor networks. The coupling capacitor prevents the input device from seeing D.C., and the input device is more effectively transferring it's voltage. This was in fact, helpful, but I still need to put more work in. 11 years since I was in college and I haven't done the math for about 8 years. 😩 *I suppose for use in guitar distortion where my goal is to overdrive the transistors and not involve diodes whatsoever, this is another path for me to explore soon after finishing current designs.* I ultimately increased my base voltage divider values by a factor of approximately 550x, which created a massive increase on input impedance and my gain in fact went through the roof. Surprisingly, the really high resistor values aren't giving me noise issues so I'm safe on the simpler path in this example, but everything is ultimately a piece of the puzzle to better performance as we progress.
How was gain Calculated where A=0.97, I understand that, how did find the value of re = 26ohm , and Capacitor value for bootstrapped to have high impedance.
There is an equation for re that is a function of emitter current and temperature. That number should be taken with a grain of salt since it can be as high as 42 ohms or as low as 21 ohms for a given current depending on what you take to be the ambient temperature. For 42 ohms the gain would be something like 0.96 instead of 0.97. The capacitor value should be set to pass all frequencies above 1Hz at the input impedance of the stage before bootstrapping. This is the impedance of the voltage divider which is about 13K, in parallel with the input impedance of the base, which is ~150K for an Hfe of 150. The 150K base impedance swamps the 4.7K resistor because it can change +/- 30% or more depending on the transistor Hfe and temperature. For a 10uF cap, this gives a rolloff of just under 1Hz.
@@thetuberoaster8321 bootstrapping the input to have greater Rin ( input impedance) . Your circuit on the first page. Your out is comming what is your collector right. So what is your gain ? With high impedance. What is your input Ac voltage : 20mv Vpp ? I am really trying to understand what's going on your video. What is the benefit of having high impedance if I am not doing audio. I am building a 3 stage amplifier CE- CE-CC. What's your thoughts on that.
@@Tankzim.z The gain is approximately Rc over Re. The intrinsic emitter resistance changes that slightly but in this case it is almost 10. The output impedance is roughly Rc which is 10K. The input impedance without bootstrapping would be dominated by the base bias network, making it hard to get anything above 10s of Kohms. Bootstrapping allows this input impedance to be at least and order of magnitude higher. This is sometimes useful for when you can't afford to load down the preceding circuit. If you have one CE stage driving another, the first will be loaded by the input impedance of the second. This will cause the gain to drop and the distortion to increase somewhat. The more obvious solution is to use a FET, but for low-noise applications this may not be feasible. This technique allows you to use a regular BJT with its benefits but without its usual trade-off of low input impedance. The input sensitivity of this amp is something like 400mV before clipping. You can tweak the biasing and the gain to change the operating point to make the input sensitivity whatever you like. The three stage amp you are talking about - the CE-CE-CC amp - allows you to split the gain between two stages, which is better in terms of distortion. The CC output will make the output much lower impedance than a normal CE amp, allowing the amp to drive heavier loads without increased distortion or loss of gain.
It's actually a rather obscure topic and I haven't found too many good sources on it. I got most of my information from a book called: Transistor Circuit Techniques by G.J. Richie the third edition. He shows a circuit where he shows exactly the techniques I describe and gives equations. I think Electronics-Tutorials online might have an article about it but it's not as good if I remember correctly. There might be more out there since I made this video.
The mushiness in the Av>80 stage wasn't about clipping (which you noticed it wasn't doing.) It is, instead, characteristic of a voltage gain that varies with the signal.
I figured it was something like that. It was actually the first time I noticed and wasn't sure what would cause that. Maybe, when the common emitter amp saturates, the bootstrap voltage gets shifted and less cancellation occurs across the resistor.
No, I wasn't intending to although Miller's Law seem like it might be helpful in a situation like this. I just found some examples of circuits in books and based my logic around that and tried not to break any rules. Actually, some of the math shown about the gain values and the exact value of the impedance is maybe not exact. I haven't found very many sources explaining these circuits that show equations to calculate gain or impedance.
The 4.7k resistor give the feedback current something to drop across. The higher that resistor is, the higher the multiplied impedance that results. The AC voltage on both sides of it are supposed to be the same. If the 4.7k resistor were just a wire, the circuit wouldn't multiply the input impedance. In fact, it would probably oscillate instead. The resistor can't be too high since the voltage divider still has to do its job of biasing the transistor.
It is somewhat arbitrary; I don't know of any calculation. Ideally it would be large enough so that the feedback capacitor doesn't have to be too large but not so big as to negate the effect of the bias network.
This is a very good explanation of bootstrapping.
Great boostrap explanation!
Great video, I tried your boostrapping gain boost but I replaced the input stage with a JFET circuit. Great tube sounding guitar boost-overdrive. Thank you!
That actually sounds like a really good idea.
Very helpful. Thank you.
Well explained and experimented. Thank you !
As I understand it, it ultimately produces a bucking voltage for your input signal so it's seeing a much higher impedance when it appears across the new input resistor networks. The coupling capacitor prevents the input device from seeing D.C., and the input device is more effectively transferring it's voltage.
This was in fact, helpful, but I still need to put more work in. 11 years since I was in college and I haven't done the math for about 8 years. 😩
*I suppose for use in guitar distortion where my goal is to overdrive the transistors and not involve diodes whatsoever, this is another path for me to explore soon after finishing current designs.* I ultimately increased my base voltage divider values by a factor of approximately 550x, which created a massive increase on input impedance and my gain in fact went through the roof. Surprisingly, the really high resistor values aren't giving me noise issues so I'm safe on the simpler path in this example, but everything is ultimately a piece of the puzzle to better performance as we progress.
Maybe the supply voltage was too small so the transistor was driven into its unlinear region?
Is this using by BJT
How was gain Calculated where A=0.97, I understand that, how did find the value of re = 26ohm , and Capacitor value for bootstrapped to have high impedance.
There is an equation for re that is a function of emitter current and temperature. That number should be taken with a grain of salt since it can be as high as 42 ohms or as low as 21 ohms for a given current depending on what you take to be the ambient temperature. For 42 ohms the gain would be something like 0.96 instead of 0.97. The capacitor value should be set to pass all frequencies above 1Hz at the input impedance of the stage before bootstrapping. This is the impedance of the voltage divider which is about 13K, in parallel with the input impedance of the base, which is ~150K for an Hfe of 150. The 150K base impedance swamps the 4.7K resistor because it can change +/- 30% or more depending on the transistor Hfe and temperature. For a 10uF cap, this gives a rolloff of just under 1Hz.
@@thetuberoaster8321 bootstrapping the input to have greater Rin ( input impedance) . Your circuit on the first page. Your out is comming what is your collector right. So what is your gain ? With high impedance.
What is your input Ac voltage : 20mv Vpp ? I am really trying to understand what's going on your video. What is the benefit of having high impedance if I am not doing audio.
I am building a 3 stage amplifier CE- CE-CC. What's your thoughts on that.
@@Tankzim.z The gain is approximately Rc over Re. The intrinsic emitter resistance changes that slightly but in this case it is almost 10. The output impedance is roughly Rc which is 10K. The input impedance without bootstrapping would be dominated by the base bias network, making it hard to get anything above 10s of Kohms. Bootstrapping allows this input impedance to be at least and order of magnitude higher. This is sometimes useful for when you can't afford to load down the preceding circuit. If you have one CE stage driving another, the first will be loaded by the input impedance of the second. This will cause the gain to drop and the distortion to increase somewhat. The more obvious solution is to use a FET, but for low-noise applications this may not be feasible. This technique allows you to use a regular BJT with its benefits but without its usual trade-off of low input impedance.
The input sensitivity of this amp is something like 400mV before clipping. You can tweak the biasing and the gain to change the operating point to make the input sensitivity whatever you like. The three stage amp you are talking about - the CE-CE-CC amp - allows you to split the gain between two stages, which is better in terms of distortion. The CC output will make the output much lower impedance than a normal CE amp, allowing the amp to drive heavier loads without increased distortion or loss of gain.
Where can one get more information on the analysis of the CE bootstrapping
It's actually a rather obscure topic and I haven't found too many good sources on it. I got most of my information from a book called: Transistor Circuit Techniques by G.J. Richie the third edition. He shows a circuit where he shows exactly the techniques I describe and gives equations. I think Electronics-Tutorials online might have an article about it but it's not as good if I remember correctly. There might be more out there since I made this video.
The mushiness in the Av>80 stage wasn't about clipping (which you noticed it wasn't doing.) It is, instead, characteristic of a voltage gain that varies with the signal.
I figured it was something like that. It was actually the first time I noticed and wasn't sure what would cause that. Maybe, when the common emitter amp saturates, the bootstrap voltage gets shifted and less cancellation occurs across the resistor.
WERE YOU USING MILLERS LAW IN THIS EXPLANATION?
No, I wasn't intending to although Miller's Law seem like it might be helpful in a situation like this. I just found some examples of circuits in books and based my logic around that and tried not to break any rules. Actually, some of the math shown about the gain values and the exact value of the impedance is maybe not exact. I haven't found very many sources explaining these circuits that show equations to calculate gain or impedance.
Why is the 4,7k resistor needed?
The 4.7k resistor give the feedback current something to drop across. The higher that resistor is, the higher the multiplied impedance that results. The AC voltage on both sides of it are supposed to be the same. If the 4.7k resistor were just a wire, the circuit wouldn't multiply the input impedance. In fact, it would probably oscillate instead. The resistor can't be too high since the voltage divider still has to do its job of biasing the transistor.
How is the 4.7k ohm calculated at 0:46?
It is somewhat arbitrary; I don't know of any calculation. Ideally it would be large enough so that the feedback capacitor doesn't have to be too large but not so big as to negate the effect of the bias network.
Thanks! 😁