Mid Boost via Emitter Degeneration?

[thehallofshields]

Lately Ive been thinking about ways to get some tone-shaping out of BJT Clipping stages, and weighing the pros and coins of the different ways I've seen.

- Feedback Loop Mid-Scoop resulting in Mid-Boost.
- Small Input Coupling Cap, Moderate Miller Capacitor

But what about Tone Shaping at the Emitter? Everyone knows the Parallel Capacitor trick for a top-boost.

Assuming the Emitter Resistor is about 1k, is there a practical way to get a Band-pass filter in there?

[Keppy]

Have you looked into gyrators? http://www.muzique.com/lab/gyrator.htm

[Keppy]

Here's the link I was looking for in the above post: http://scopeboy.com/elec/gyrator.html

Basically, a gyrator simulates an inductor to ground. The link in this pose shows the equivalency of the gyrator to an inductor. The link in the post above adds a capacitor to show how to simulate an LC bandpass filter.

The issue with this approach is that there is some series resistance, so it may not function well in parallel with 1k. You'll have to experiment.

[thehallofshields]

That's incredible. I can't believe I've never seen that before.

I really wish the musique page had a graph output. I'll have to read up on Inductors to understand Q.

[Gus]

This has a low pass after the gain stage.

[R.G.]

Not to be a spoil-sport, but the necessary housekeeping will almost certainly push the circuit to do mid boost from an emitter into being more parts and more complicated than some thing simple like a single opamp bandpass or a Twin T.

The Twin T from collector to base will do the job nicely and has a fixed number of components and a way to adjust both tuning and Q in a straighforward way.

[thehallofshields]

Not to be a spoil-sport, but the necessary housekeeping will almost certainly push the circuit to do mid boost from an emitter into being more parts and more complicated than some thing simple like a single opamp bandpass or a Twin T.

The Twin T from collector to base will do the job nicely and has a fixed number of components and a way to adjust both tuning and Q in a straighforward way.

Thanks, That's what I expected.

What is a Twin T?

[thehallofshields]

So do I treat this like: Inductor->56kOhm->Ground?

[Keppy]

What is a Twin T?

http://www.geofex.com/Article_Folders/EQs/paramet.htm

So do I treat this like: Inductor->56kOhm->Ground?

Answer from Wikipedia, referring to this picture:

In the circuit shown, one port of the gyrator is between the input terminal and ground, while the other port is terminated with the capacitor. The circuit works by inverting and multiplying the effect of the capacitor in an RC differentiating circuit where the voltage across the resistor behaves through time in the same manner as the voltage across an inductor. The op-amp follower buffers this voltage and applies it back to the input through the resistor RL. The desired effect is an impedance of the form of an ideal inductor L with a series resistance RL:

Z = R_\mathrm{L} + j \omega L \,\!

From the diagram, the input impedance of the op-amp circuit is:

Z_\mathrm{in} = \left(   R_\mathrm{L} + j \omega R_\mathrm{L} R C \right) \| \left( R + {1 \over {j \omega C}} \right)

With RLRC = L, it can be seen that the impedance of the simulated inductor is the desired impedance in parallel with the impedance of the RC circuit. In typical designs, R is chosen to be sufficiently large such that the first term dominates; thus, the RC circuit's impact on input impedance is negligible.

Z_\mathrm{in} = R_\mathrm{L} + j \omega R_\mathrm{L} R C \,\!

This is the same as a resistance RL in series with an inductance L = RLRC. There is a practical limit on the minimum value that RL can take, determined by the current output capability of the op amp.

RL corresponds to R1 in the picture you posted, so it should be treated as R1->Inductor->Ground. R1 would have to be significantly less than 1k to allow any effect in parallel with a 1k emitter resistor. Like R.G. said, this isn't the most practical way to do this. But it is a way.

Lately Ive been thinking about ways to get some tone-shaping out of BJT Clipping stages, and weighing the pros and coins of the different ways I've seen.

- Feedback Loop Mid-Scoop resulting in Mid-Boost.
- Small Input Coupling Cap, Moderate Miller Capacitor

But what about Tone Shaping at the Emitter? Everyone knows the Parallel Capacitor trick for a top-boost.

Assuming the Emitter Resistor is about 1k, is there a practical way to get a Band-pass filter in there?

What about an emitter bypass cap combined with the Miller cap?

The Twin T from collector to base will do the job nicely and has a fixed number of components and a way to adjust both tuning and Q in a straighforward way.

Way to kill the fun with a practical suggestion. Is there anything to prevent this from being used as an emitter bypass instead of collector to base? Maybe not in this case, since the filter would be in parallel with the 1k emitter resistor and it would be difficult to get the filter impedance low enough. If it could be made to work, though, the potential advantage I see is greater gain from the stage, since an emitter bypass raises the gain at the desired frequencies and the collector-bass feedback reduces gain at all other frequencies.

[R.G.]

Is there anything to prevent this from being used as an emitter bypass instead of collector to base? Maybe not in this case, since the filter would be in parallel with the 1k emitter resistor and it would be difficult to get the filter impedance low enough. If it could be made to work, though, the potential advantage I see is greater gain from the stage, since an emitter bypass raises the gain at the desired frequencies and the collector-bass feedback reduces gain at all other frequencies.

Hmmm.

It will be tough getting the overall impedance of the Twin T stage low enough to be effective. The cap values will be pretty big. And the Twin T is the wrong impedance-gender. A Twin T has a high impedance at the tuning frequency, lower everywhere else. So if it's across an emitter resistor, it forces higher gain everywhere except at the tuned frequency, making the overall gain be a notch, not a peak.

If an emitter shunt is the desired implemention, a series L-C is what you want, all right. A capacitor and emitter-follower-gyrator will do the job, but will require tinkering. This will give a shunt of the 1K resistor at the desired frequency and lower gain elsewhere. The circuit for one of the gyrator/LC sections from a graphic EQ at the link you posted will do. Here's just the pic: http://www.geofex.com/Article_Folders/EQs/greq.gif

[bool]

I'm not sure what would be wrong with a "top-boost" emmiter shunt cap in combination with either a) well proportioned miller cap or b) collector shunt cap to gnd. Well, you could combine both but that's a waste imho.

Just works, you know? Cheap, too.

[Gus]

Bool
Did you look at what I posted?  The circuit has a controlled gain change set by R5, C3.  The treble/mid boost is then low pass filtered R6, R7, C5 and...
The input is bootstrapped

[bool]

Sure, Gus ... but you're waisting too much components for it to go into mass production ...

[Keppy]

A Twin T has a high impedance at the tuning frequency, lower everywhere else.

Really? I thought the opposite. Guess I'll have to do some more reading.

I thought that being a notch filter, the tuning frequency was shunted to ground, which we'd want in an emitter bypass to boost the tuning frequency.