really fast buffers

How does an emitter follower do anything useful if the base is tied to ground?

I said "bypassed to ground." But an emitter follower can certainly work, and oscillate, if its base is really grounded.

Nice, but it's all low-voltage differential. I need 5 volt swings in and out.

My DUT will be spec'd to 1 ns accuracy (well, the customer wants 200 ps, but that's silly) and maybe 30 ps RMS jitter. We'll have to occasionally characterize all the component and traces and cable delays in the test set, and fudge them out of the tests. Gigantic pain. After that, we'd like things to be stable to maybe 100 ps.

I guess I should put an RTD or an LM35 on the test board, so my interfaced DVM can measure board temperature. I sure hope we'd never need to actually use that.

How do you make an emitter follower when the base is bypassed to ground? Where do you put the signal?

How? There's no signal on the base.

It can certainly oscillate. But this is one place you would add a series base resistor to kill any parasitic oscillations.

Then you find you have a problem in the field. You have to rework all the units you have shipped. Why not add a suitable base suppression resistor to critical circuits in the first place.

I specified a value that does not degrade performance.

There is absolutely no value in having a wider bandwith than you really need.

Am 05.10.2018 um 02:23 schrieb John Larkin:

A real pain. And the receiver inputs after that will probably switch at

A customer of mine measured their cables in the temp chamber. That was real "fun".

where it hurts most, esp. when most quality cables are PTFE.

If you want stable delays, it's probably best to try to be as fast as possible and then limit the BW with film R and NP0 C to sane values. Excess BW brings excess noise, brings excess jitter.

Distributing a sine wave + a qualifier for the zero crossings takes cable dispersion out of the equation. One gains precisely located points in time but loses arbitrary edge position.

regards, Gerhard

In your circuits, maybe. I often care very much about fractional nanovolts of noise.

In an audio amp, very true. In instruments, not so much. One of my main support headaches is people using the log output of the laser noise canceller, and adjusting the feedback bandwidth to set their measurement bandwidth. Causes all sorts of mayhem when the beam power ratios vary by order unity at frequencies near the feedback bandwidth. Running it fast fixes that, and it's easy to put an RC on the output if you like.

It's usually best to have lots of bandwidth and then put in a good quality filter near the end. For instance, several of my gizmos use THS3091 CFAs as the output stage, configured as a two-pole Sallen-Key lowpass, and with a 50-ohm resistor in series with the output. The amp is 100 times faster than some of those circuits need, but it's super beefy (and hence student-resistant), and the extra speed keeps the S-K rolloff well controlled.

Or it might have lower beta, in which case your procedure has the same problem.

I generally go for about a factor of 1.5x to 2x more resistance than needed to cure the oscillation, and have never had an oscillation problem in the field.

Cheers

Phil Hobbs

The classical example is the one-transistor !RESET generator, which is a slow RC with an emitter follower.

Cascading PNP and NPN followers to compensate V_BE is also a good way of making an oscillator.

Cheers

Phil Hobbs

Yup. TC of epsilon is about 2000 ppm/K in the glass transition region iirc. Polyethylene is much better.

Interesting idea.

Cheers

Phil Hobbs

Oh, and the cap multiplier, of course. They can oscillate if you use too fast a transistor.

Cheers

Phil Hobbs

Just pull the emitter negative with a resistor or a current sink. That can make a nice UHF oscillator.

Exactly. That's the first time I saw an emitter follower oscillate, an RC+emitter follower+74xx schmitt. The base hung and never charged up enough to trip the schmitt; the RF used too much base current or something. That confused me for a while. 2N2219 loved to oscillate.

Never seen that; good to know.

Might as well just ground the base. But that's not an emitter follower, as you stated in your first post. There's no place to put a signal.

Just put a proper value resistor in series with the base. It won't oscillate. But it's still not an emitter follower.

It oscillates because of a slow edge and no hysteresis?

Is it similar to cap. multiplier oscillations? (do all single transistor circuits map onto each other?)

George H.

Or a Sallen-Key filter.

The transistor itself can oscillate, typically around 100 MHz, in the emitter follower config, if the base is RF bypassed to ground. A bit of resistance in series with the base, 33 or 50 ohms or something, kills it. That adds some noise, Johnson noise and Ib shot noise current into the resistor.

I don't use bipolar transistors much any more.

It oscillates because of inductance in the base lead. Along with the base- emitter capacitance and capacitance from the emitter to ground, this forms a Colpitts oscillator, which is one of the most vigorous oscillators known. It can accept an extremely wide range of parameters and still oscillate.

The purpose of the resistor in the base lead is to dampen the Q of the tank so oscillations are no longer possible.

Could be. Any place where you have some inductance in the base and capacitance from the emitter to ground could form a Colpitts, especially where you have some inductance in the emitter lead.

Other configurations where fast transistors are connected together could become an oscillator. Some examples are:

Cascode Darlington Sziklai pair etc

The various configurations of the Colpitts are basically the same. It's just where you put the ground that makes them look different.

The difficulty with the Colpitts is it loves to oscillate. The conditions are layout-dependent and dependent on the gain and bandwidth of the transistor. The oscillations can completely destroy the desired performance of the circuit.

My approach is to assume the circuit will oscillate under the right conditions, and to include a resistor in the base in all critical circuits that could be susceptible to parasitics. After a while, you get to know what value would be appropriate, usually from 5 to 50 ohms. But check carefully.

This creates a problem. Often the oscillation frequency is far above the frequency range of the best equipment available in your lab. To compensate, you need some way to perturb the circuit and look for any change in the operating parameters. One method is to use a dental pick which is available at most pharmacies. This is a small stainless hook imbedded in plastic, with a cover to fit over the top. You can touch various points in the circuit an see if you can detect any change in the operataion.

The best by far is a wideband spectrum analyzer. You can make a small loop feeding a coax cable to the analyzer. Bring the loop near the circuit. any oscillation should become visible in the display.

Of course, with modern transistors, especially SiGe, you need a very high frequecy specctrum analyzer. These are not cheap.

Yep, for a bit lower Ft transistors my FM radio warns me:-)

I have, and this is REAL fun, a bug detector from ebay, it will beep if any RF in the GHz range is detected, very similar to this one:

You can test your microwave's leakage with it too.

I build some simple detectors uses microwave diodes too.