the ultimate PNP transistor

why not change the design of the base drive so it is not saturated,,,, it will switch off much faster... or apply a positve voltage (current actually) to turn it off faster...

Mark

Yet another way to pull charges out of the base is to use a resistor from input drive to base of PNP, the drive to the base of an inverted NPN: collector to ground and emitter to base of PNP. No additinal supplies needed.

Use anti-saturation diodes or - if that does not work - two transistors in a Cascode connection.

The designation should have read BFT92. An obsolete TO-72 version was called BFQ52.

Anti-saturation diodes would *add* capacitance.. But cascode operation would definitely kill the miller effect (on both transistors).

in a

The "miller capacitance" the O.P. have problems with is (roughly): the charge stored in the base-collector region + charge in the physical capacitance of the base junction and then the whole thing is multiplied by the voltage swing of the collector.

So adding a little more physical capacitance might bring an overall improvement in performance by reducing the base-collector charge which is penalised extra by the collector voltage swing.

By the elimination of the collector voltage change in the driver transistor - the top transistor still gets the full whack BUT the base is fed from a low impedance source so one does not see it. The stored charge comes out through the base when the top transistor switches off - for fast switching it is neccessary to decouple the base of the top transistor well.

skrev i melding news: snipped-for-privacy@o13g2000cwo.googlegroups.com...

What about this little design-trick:

o || | .----||----. | | || | | | | | | ___ | |/ o-------o--|___|---o----| | |>

| | | | - | ^ | | | | | === === GND GND (created by AACircuit v1.28 beta 10/06/04

Geir Klemetsen schrieb:

As I've mentioned somewhere else already, for BFT52 read BFT92.

My drive is a current source with little head room. So I'm just feeding

the current into the base of the PNP transistor acting as a "digitizing

current mirror". The mark-space ratio is determined by a resistor from base to collector. With a BF324 (zero-bias Cbc ~ 3pF, beta ~ 50) at 1mA

collector current, this works only up to a few MHz. Easy to simulate with Spice - but be sure to get the CJC right!

VCC o-------------o------o 9V | | |=AF| | | | | Rb |_| | | |< current in >------o----| BF324, BF450, BFQ52, etc |\ | o------> voltage out | CMOS load ~ 5pF |=AF| | | |_| Rc | 0V o---------------------o =20

Martin.

a écrit dans le message de news: snipped-for-privacy@g49g2000cwa.googlegroups.com...

Geir Klemetsen schrieb:

As I've mentioned somewhere else already, for BFT52 read BFT92.

My drive is a current source with little head room. So I'm just feeding

the current into the base of the PNP transistor acting as a "digitizing

current mirror". The mark-space ratio is determined by a resistor from base to collector. With a BF324 (zero-bias Cbc ~ 3pF, beta ~ 50) at 1mA

collector current, this works only up to a few MHz. Easy to simulate with Spice - but be sure to get the CJC right!

VCC o-------------o------o 9V | | |¯| | | | | Rb |_| | | |< current in >------o----| BF324, BF450, BFQ52, etc |\ | o------> voltage out | CMOS load ~ 5pF |¯| | | |_| Rc | 0V o---------------------o

Martin.

Martin, if your source is a current source then Cbc is indeed the limiting factor in this circuit.

What are your parameters? From your 9V/1mA conditions I expect Rc to be 9K (probably 10K or 8.2K). This is already 50ns with your 5pF load parasitics, and doesn't include any Miller effect at all.

With a current source base driving you'll have to go the cascode route.

Before going any further, what are your circuit parameters (base current source, power supply allowed current, expected output 10-90% or whatever rise time, expected output swing)?

Fred Bartoli schrieb:

or

y=2E

ce

^^^^^^^^^ emitter, of course!

in

s,

My current working circuitry corresponds to this Spice code:

• PNP digitizing current mirror
• v0 1 0 9 ! 9V battery
• ii 2 0 sin(0.6m 0.4m 4Meg) ! 0.2 to 1.0mA sinusoidal modulation
• qp 3 2 1 BFT92 ! CJC as small as possible (~1p) rb 2 1 1.1k ! adjust value if necessary rc 3 0 4.7k ! 1mA average co 3 0 5p ! CMOS gate input (ST: 5p typ, 7.5p max)

• Philips model, Version: 2.0, Date: July 1992
• .model BFT92 PNP(IS=3D4.37563E-16 BF=3D33.5815 NF=3D1.00972 VAF=3D23.3946
• IKF=3D9.95381E-2 ISE=3D8.70539E-14 NE=3D1.94395 BR=3D4.94721 NR=3D1.00254
• VAR=3D3.90385 IKR=3D5.28157E-3 ISC=3D3.58864E-14 NC=3D1.39333 RB=3D5 IRB= =3D1E-6
• RBM=3D5 RE=3D1 RC=3D10 EG=3D1.11 XTI=3D3 CJE=3D7.46659E-13 VJE=3D0.6 MJE= =3D0.356829
• TF=3D1.74921E-11 XTF=3D1.35455 VTF=3D0.155654 ITF=3D1E-3 PTF=3D45
• CJC=3D9.37103E-13 VJC=3D0.396455 MJC=3D0.199949)

Although I have included the model of the 5GHz BFT92 (SOT23 package), my present circuit uses the 450MHz BF324 (TO-92 package). At 4MHz, the signal rise (as observed with a probe capacitance

That's no surprise, with the 2n5771's relatively high capacitances, 3pF, compared a much lower 0.3 and 0.55pF at 5-10V reverse bias.

About 0.7pF at sensible reverse bias according to the datasheets, just a bit higher than you've been using. But there are plenty of lower-capacitance smaller-die-size choices if that's important.

Sheesh!

DOUBLE-SHEEEEESH! Such amateurs ;-)

...Jim Thompson

What a minute. First I invent Sheesh!, then you go and invent DOUBLE-SHEEEEESH!? Sheesh!

Win, Did you really invent "she..esh"?

I've had it in my spelling checker... many different lengths of e-strings, for a very very long time ;-)

Same with "Bwahahaha..haha", no matter the number of rat-a-tats, it always passes spelling check ;-)

...Jim Thompson

Of course not. IIRC, my dad used to use the term, so it must go way back. I imagine it's a polite way of saying s*it.

Come to think of it, I _do_ remember my mother saying "sheeesh" ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

Cripes, no. The American Heritage dictionary says it's an alteration of "Jesus".

Best regards, Spehro Pefhany

What, a religious swear word, taking the Lord's name in vain? S*it!

I actually thought it was another form of "Jesus". Like "Holy Mackarel" or something.