emitter follower gain

Here's a simple emitter follower:

Version 4 SHEET 1 880 680 WIRE 272 -112 224 -112 WIRE 368 -112 272 -112 WIRE 368 -64 368 -112 WIRE 368 64 368 16 WIRE 224 96 224 -112 WIRE -32 144 -112 144 WIRE 112 144 48 144 WIRE 128 144 112 144 WIRE 160 144 128 144 WIRE 464 176 400 176 WIRE 528 176 464 176 WIRE -112 208 -112 144 WIRE 400 224 400 176 WIRE 128 240 128 144 WIRE 352 240 128 240 WIRE 224 288 224 192 WIRE 272 288 224 288 WIRE 352 288 272 288 WIRE -112 336 -112 288 WIRE 224 336 224 288 WIRE 400 352 400 304 WIRE 224 480 224 416 FLAG 224 480 0 FLAG 368 64 0 FLAG -112 336 0 FLAG 400 352 0 FLAG 464 176 ERR FLAG 272 -112 C FLAG 112 144 B FLAG 272 288 E SYMBOL npn 160 96 R0 WINDOW 0 95 31 Left 2 WINDOW 3 73 62 Left 2 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL current 224 336 R0 SYMATTR InstName I1 SYMATTR Value 10m SYMBOL voltage 368 -80 R0 SYMATTR InstName V1 SYMATTR Value 10 SYMBOL voltage -112 192 R0 WINDOW 0 48 57 Left 2 WINDOW 3 41 96 Left 2 WINDOW 123 43 129 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value SINE(0 0.5 1) SYMATTR Value2 AC 1 SYMBOL res 64 128 R90 WINDOW 0 -60 55 VBottom 2 WINDOW 3 -49 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 1m SYMBOL e 400 208 R0 WINDOW 0 51 53 Left 2 WINDOW 3 56 87 Left 2 SYMATTR InstName E1 SYMATTR Value 1 TEXT -32 -16 Left 2 !.tran 10 TEXT -96 -104 Left 2 ;Emitter Follower Gain TEXT -64 -64 Left 2 ;JL April 4, 2015

Spice reports the error as 270 uV p-p (ie, 270 PPM, gain = 0.99973). The base input impedance looks like about 2.2 megs. Is that gain realistic?

The LT Spice 2N4401 model has the forward Early voltage Vaf=90.7, and no reverse Early voltage, whatever that is. The 90 volts is actually kinda low; some NPNs are over 1000.

AoE3 reports the measured 2N4401 Ve as 410.

This gain would be a mild nuisance to measure.

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 
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John Larkin
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Not too bad actually--try bootstrapping a 10 nF capacitor with a 10k resistor to a (slow) pulser, and measure the rise time. The low frequency gain is

A_V = 1-t_R/t_RC,

where t_RC is without the bootstrap, and t_r is with.

Some of the new NXP RF transistors, e.g. the BFU5xx series, have pretty decent Early voltages for 9 GHz silicon devices--much better than the older ones like the BFR505. And SiGe:C BJTs have effectively infinite VAF. (I keep going on about that, I know, but it's dead useful.)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

I have a 547 scope with a super differential amp plugin, so I could measure the error directly, just like the Spice sim. Or maybe transformer couple the error voltage.

But if the sim is anything like realistic, the gain is good enough.

According to Win, ZXTN2018 has a measured Vf of 4600!

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John Larkin         Highland Technology, Inc 
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John Larkin

Probably. You bias the transistor with a perfect current source, and have an infinite load. Under those conditions the change in Vbe would be very small. Our E'qn 2.14 says it's n (actually Eta) times the signal size. n is roughly V_T / V_A = 25mV / 90.7V = 275 ppm.

I'm not a big LTSpice user, where does it show you its component parameters?

That's our measured value of the Early voltage, V_A.

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    - Win
Reply to
Winfield Hill

Snitch his reel. ;)

Cheers

Phil Hobbs

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Phil Hobbs

I get follower gains of 0.9997ish relatively routinely in real circuits for clients--generally < 1MHz bootstraps for giant photodiodes with very low photocurrents. BF862 with series feedback (e.g. from a wraparound PNP, as in the old-time current boosters for linear regulators), good quality current sink, bootstrapped drain.

One was for a 1600 pF photodiode with a 200 nA full scale range, shot-noise limited with a 100 Mohm feedback resistor up to (some reasonable frequency that the customer was very happy with). You really really need a good bootstrap with that setup!

The nice thing about those is that all the improvements are inside the local feedback loop controlled by the BF862, so there's virtually no noise penalty, which there so often is otherwise.

Cheers

Phil Hobbs

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Phil Hobbs

In associated .LIB or .BJT files.

Beware that Mikey is fond of taking slights-of-hand... his default .MODEL D is _not_ the same as PSpice's default .MODEL D, but Mikey is currently making great claims touting LTspice over PSpice based on that.

I think there must be a pissing contest going on between the Cadence/OrCAD PSpice folks and the LTspice folks... all a bunch of foolishness comparing apples and oranges ;-)

Beginners should take note that the value of VA is the negative voltage axis intercept of the IC vs VCE slope. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Jim Thompson

The high-current low-saturation-voltage BJTs that Zetex makes, to compete with MOSFETs, have a number of unusual & interesting properties. This one has a nice low r_bb' of 3.3 ohms, and a high datasheet beta of 200 up to several amps, even with Vce = 1V. But its beta deteriorates badly at Ic below 1mA.

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Reply to
Winfield Hill

That's curve 28 in Figure 8.39 (the figure has 44 plots of common "small-signal" transistors). Paul and I each purchased a two-channel SMU, a Keithley 2612A for me (does pulsed meas to 10A), and an Agilent for him. I selected and purchased piles of transistors and we went mad taking measurements.

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Winfield Hill

I wouldn't have considered a 5 amp transistor for my small-signal application, but that one looks like the winner.

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John Larkin         Highland Technology, Inc 
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John Larkin

You didn't use scut bunnies?

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John Larkin         Highland Technology, Inc 
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John Larkin

Pretty close agreement with LT Spice. That's good.

Sure, I meant the same thing. Typing accurately is not part of my job description.

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John Larkin         Highland Technology, Inc 
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John Larkin

How does Vbe vary from about 0.4v to 1v for this part?

NT

Reply to
meow2222

It follows the usual temperature and current diode equation, but at high currents, like 10 amps (in a SOT23!) the emitter resistance starts to drop a bunch of voltage (wire bonds, ohmic effects on chip) so the apparent Vbe goes up. You can see on the data sheet, where the Vbe stops being logarithmic and starts being linear/ohmic.

The base resistance Rbb' is 3.3 ohms, so that starts to matter too, at high currents. At 10 amps, HFE is about 30, so the base current is about 300 mA, which calculates to a full volt of drop in Rbb'.

It's pretty conventional up to 1 amp Ic.

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John Larkin         Highland Technology, Inc 
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John Larkin

Right-click on the transistor symbol, and select "pick new transistor." The pulldown list shows the available transistors and their params.

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John Larkin         Highland Technology, Inc 
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John Larkin

DigiKey, 35 cents each, 1k. Competes well with MOSFETs.

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 Thanks, 
    - Win
Reply to
Winfield Hill

Your SPICE design had a perfect current source and infinite-impedance load, not a usual circumstance. Real circuits are different, and emitter-followers can benefit from either-or-both Sziklai or cascode configurations, and the addition of an op-amp. Or maybe you want a JFET source follower. These can do very well in the Sziklai configuration.

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    - Win
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Winfield Hill

They also have remarkably good inverted hFE (BR within 50% of BF) -- a necessary consequence of low Vce(sat) I think.

The SPICE models usually lie grossly about this parameter, not sure why (aside from obviously not being characterised for that operation).

Tim

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Tim Williams

But I'm going to do that! Almost.

I want super low noise, at about 10 mA.

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John Larkin         Highland Technology, Inc 
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John Larkin

Interesting. At 10mA re = 2.5 ohms, its contribution to the voltage noise en, expressed as noise resistance, is 1.25 ohms. That means the 3.3-ohm r_bb' dominates the voltage noise, see E'qn 8.20, ignoring base-current shot noise. And it means you could lower Ic to say 5mA, without hurting e_n. Alternately, if you want to spend 10mA anyway, dividing it among two parallel transistors would be a good idea, pushing the r_bb' contribution down to 2.3 ohms, and you're in the 0.2nV territory.

We played that game with a bunch of ZTX951 transistors, pushing e_n of a differential amplifier down to 0.07nV, see Figure 8.45. Numbers like may be best expressed in picovolts, i.e., 70pV. :-)

But all of this assumes your signal's source resistance is very low, otherwise why go to all the extra work?

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    - Win
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Winfield Hill

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