Fast NPN/PNP transistors for a totempole driver

[Klaus Kragelund]

Hi

I have a 74HCT14 hex inverter driving a NPN/PNP (BC847/BC857) totempole
stage connected to GND and 5V

The output is a small capacitor connected from the common node to
ground

The bottom PNP transistor has a current sense resistor to ground so I
can measure the current when the output falls from 5V to zero

My problem is I want to drive the totempole with about 10ns tr/tf and
this causes an error current in the PNP transistor due to BE parasitic
capacitances

So I need a couple of fast transistors - better than the BC857 with
regards to capacitance and ft and it needs to be affordable. Anyone got
a direct hit?

Regards

Klaus

 

[[email protected]]

Hi

I have a 74HCT14 hex inverter driving a NPN/PNP (BC847/BC857) totempole
stage connected to GND and 5V

The output is a small capacitor connected from the common node to
ground

The bottom PNP transistor has a current sense resistor to ground so I
can measure the current when the output falls from 5V to zero

My problem is I want to drive the totempole with about 10ns tr/tf and
this causes an error current in the PNP transistor due to BE parasitic
capacitances

So I need a couple of fast transistors - better than the BC857 with
regards to capacitance and ft and it needs to be affordable. Anyone got
a direct hit?

BFR92/BFT92 - 5GHz at 14mA. Check out the prices from Farnell - a euro
or so IIRR.

 

[Marte Schwarz]

Hi Klaus,

I have a 74HCT14 hex inverter driving a NPN/PNP (BC847/BC857) totempole
stage connected to GND and 5V

I would test 74ACT14.

Marte

 

[[email protected]]

Hi Klaus,


I would test 74ACT14.

Marte

Yep. Or even parallel a few inveters for more current, all depending
on how heavy the load is.

James Arthur

 

[Klaus Vestergaard Kragelund]

Yep. Or even parallel a few inveters for more current, all depending
on how heavy the load is.

James Arthur

I did infact look at the ACT devices, but could not right away find info
if its ok to parallel them since the device is a schmitt trigger and a
slow changing input voltage would lead to much "cross" conduction
between devices. Ofcourse I could use one device to sharpen the input
edge and run the others off this first inverter. Is it ok to parallel if
the cross conduction period is small (perhaps with output resistors on
all outputs)?

Moreover I read one this group a while ago about a special part rated
for high currents (perhaps it was a 4000 series chip). I have searched
but could not find the thread......

Would it be possible to use another coupling to get the speed and low
parasitic capacitance I'm after? And still being able to measure by
means of a current sense resistor the transient current in an unknown
capacitor?

Regards

Klaus

 

[[email protected]]

I did infact look at the ACT devices, but could not right away find info
if its ok to parallel them since the device is a schmitt trigger and a
slow changing input voltage would lead to much "cross" conduction
between devices. Ofcourse I could use one device to sharpen the input
edge and run the others off this first inverter. Is it ok to parallel if
the cross conduction period is small (perhaps with output resistors on
all outputs)?

Moreover I read one this group a while ago about a special part rated
for high currents (perhaps it was a 4000 series chip). I have searched
but could not find the thread......

Would it be possible to use another coupling to get the speed and low
parasitic capacitance I'm after? And still being able to measure by
means of a current sense resistor the transient current in an unknown
capacitor?

Regards

Klaus

Yes, you can parallel them, and yes, you can expect them to
cross-conduct for a fraction of a nS, drawing a nasty spike. It's not
destructive though, so you just bypass and ground accordingly, or add
current-sharing resistors if you really care.

Squaring up the waveform with a preliminary inverter is a good idea,
and I'd square it up with something before the 'ACT14, personally. I
don't know about the 'ACT14 in particular, but I've seen grossly
excessive dissipation in earlier devices when fed slow waveforms.

Another possiblity is using a MOSFET gate driver. Those are
basically very hefty CMOS inverters, some including bipolar buffers on
their outputs. I've got some TPSxxxx devices from T.I. fitting that
description.

Cheers,
James Arthur

 

[Robert Baer]

Hi

I have a 74HCT14 hex inverter driving a NPN/PNP (BC847/BC857) totempole
stage connected to GND and 5V

The output is a small capacitor connected from the common node to
ground

The bottom PNP transistor has a current sense resistor to ground so I
can measure the current when the output falls from 5V to zero

My problem is I want to drive the totempole with about 10ns tr/tf and
this causes an error current in the PNP transistor due to BE parasitic
capacitances

So I need a couple of fast transistors - better than the BC857 with
regards to capacitance and ft and it needs to be affordable. Anyone got
a direct hit?

Regards

Klaus

Take a look at the schematic of the "standard" NPN/NPN totempole
output used in the 74xx series of logic.
There is a short period of time (up to 6nSec) when both the top and
bottom NPNs conduct, creating the "sharp spike" that others mentioned
elsewhere in this thread.
That happens when the bottom NPN is supposed to turn off, and the top
NPN is turned on.
That happens *not* due to B-E capacitance that you refer to, but to
stored charge in the base that must be removed for the NPN to turn off.
Perhaps a similar problem exists inyour (discree?) circuit.
The solution to the NPN/NPN totem pole problem is to 1) put a
resistor in series with the base of that bottom NPN, and 2) add a PNP
with its emitter connected to the NPN base and its base to the "input"
end of the resistor.
When the driver turns off, the PNP turns on and pulls the base charge
out.
This scheme can speed up the output from 2 to 5nSec and will
completely eliminate the spike at most temperatures (in an IC, the spike
is reduced by only a few orders of magnitude at 125C).

 

[Rene Tschaggelar]

Hi

I have a 74HCT14 hex inverter driving a NPN/PNP (BC847/BC857) totempole
stage connected to GND and 5V

The output is a small capacitor connected from the common node to
ground

The bottom PNP transistor has a current sense resistor to ground so I
can measure the current when the output falls from 5V to zero

My problem is I want to drive the totempole with about 10ns tr/tf and
this causes an error current in the PNP transistor due to BE parasitic
capacitances

So I need a couple of fast transistors - better than the BC857 with
regards to capacitance and ft and it needs to be affordable. Anyone got
a direct hit?

Klaus,
forget TTL, it was never designed to cope with 100MHz.
Have a look at other families.
What kind of voltage range on the output is required
and what is the load.

Rene

 

[Klaus Kragelund]

Klaus,
forget TTL, it was never designed to cope with 100MHz.
Have a look at other families.
What kind of voltage range on the output is required
and what is the load.

The output load from the totempole point of view is >300Ohm and max
100pF

5V or any other voltage will be fine

Regards

Klaus

 

[[email protected]]

The output load from the totempole point of view is >300Ohm and max
100pF

5V or any other voltage will be fine

Regards
Klaus

100pF raised 5 volts in 10nS will take 50mA, a very modest
requirement. Paralleled 74ACxxxx CMOS gates will do the job quite
easily, and make pretty rail-to-rail squarewaves too.

Even the 74HCxxxx types might do--a single 74HC244 is guaranteed to
drive 50pF in 12nS--but you'd need to parallel more gates than with the
newer stuff.

Regards,
James Arthur

 

[Rene Tschaggelar]

The output load from the totempole point of view is >300Ohm and max
100pF

5V or any other voltage will be fine

I tend to use the 74F0037, a quad NAND driver with 30 Ohms output
impedance. It rises to 2.5V in 2ns then becomes slower, say another
4ns to 4V, all into 50 Ohms.

Then these are MOS driversfrom various manufacturers.

Rene

 

[Ken Smith]

Even the 74HCxxxx types might do--a single 74HC244 is guaranteed to
drive 50pF in 12nS--but you'd need to parallel more gates than with the
newer stuff.

If you want faster edges, you can run the thing on 5.5V. They get a bit
faster as you go up from the STD 5V supply.

 

[Jim Thompson]

If you want faster edges, you can run the thing on 5.5V. They get a bit
faster as you go up from the STD 5V supply.

--

I think this thread has gone astray. The OP wanted to measure load
current, which was being obscured by totem-pole device capacitive
currents.

...Jim Thompson

 

[Ken Smith]

I think this thread has gone astray. The OP wanted to measure load
current, which was being obscured by totem-pole device capacitive
currents.

Not astray just digressing wildly.

 

[[email protected]]

I think this thread has gone astray. The OP wanted to measure load
current, which was being obscured by totem-pole device capacitive
currents.

...Jim Thompson

Hmm... I took the OP's current-sense resistor to be for
troubleshooting, but you could be right Jim.

No worries, just move the sense resistor to the CMOS gate's Vss. Be
sure to feed the CMOS driver-part fast edges though, and/or use a
non-inverting part to avoid negative-feedback via Vss & such. (a small
Vss resistor + output resistor adds a little input hysteresis to an
ordinary, non-inverting CMOS gate.)

I suggested paralleling CMOS gates because the OP indicated he's
already using one as the driver.

Cheers,
James Arthur

 

[Fred Bloggs]

I have a 74HCT14 hex inverter driving a NPN/PNP (BC847/BC857) totempole
stage connected to GND and 5V

Excuse me...but a PNP/NPN combo is not a "totempole." Who the hell knows
what this person is asking? I hope he communicates to himself better
than this- he will be stuck forever.

[snip]

 

[[email protected]]

I have a 74HCT14 hex inverter driving a NPN/PNP (BC847/BC857) totempole
stage connected to GND and 5V

Excuse me...but a PNP/NPN combo is not a "totempole." Who the hell knows
what this person is asking? I hope he communicates to himself better
than this- he will be stuck forever.

[snip]

Yeah, you're right, it's not what we normally mean by 'totem pole,'
but from this:

"The bottom PNP transistor has a current sense resistor to ground so I
can measure the current when the output falls from 5V to zero "

I took the OP to mean this:

.. Vcc
.. -+-
.. |
.. |
.. |/ Q1
.. +---| BC847
.. | |>.
.. | |
.. | o----> >---o----+
.. |\ | | | |
.. | \ | |<' C(L) --- .-.
.. --| >O--o---| 100pF --- | | R(L) = 300 ohms
.. | / |\ | | |
.. |/ Q2 | === '-'
.. BC857 | |
.. 'HCT14 .-. ===
.. | |
.. | | Rs
.. '-'
.. |
.. |
.. ===
.. GND

Best Regards,
James Arthur

 

[Winfield Hill]

[email protected] wrote...

Fred Bloggs wrote:

Yeah, you're right, it's not what we normally mean by 'totem pole,'

I took the OP to mean this:

. Vcc
. -+-
. |
. |
. |/ Q1
. +---| BC847
. | |>.
. | |
. | o----> >---o----+
. |\ | | | |
. | \ | |<' C(L) --- .-.
. --| >O--o---| 100pF --- | | R(L) = 300 ohms
. | / |\ | | |
. |/ Q2 | === '-'
. BC857 | |
. 'HCT14 .-. ===
. | |
. | | Rs
. '-'
. |
. |
. ===
. GND

Right. Whereas a totem pole is this:

. Vcc
. -+-
. |
. |/ Q1
. +----| BC847
. | |>. etc
. | |
. | o----> >---o----+
. |\ | | | |
. | \ | | C(L) --- .-.
. --| >O--o--|<|-' 100pF --- | | R(L) = 300 ohms
. | / | | |
. |/ === '-'
. |
. 'HCT14 ===

Which is much more simple, but depends on the driver's pulldown
capability for the sinking current. It's especially well suited
for designs with high-performance NPN or N-channel devices.

 

[Klaus Vestergaard Kragelund]

[....]

Even the 74HCxxxx types might do--a single 74HC244 is guaranteed to
drive 50pF in 12nS--but you'd need to parallel more gates than with the
newer stuff.

If you want faster edges, you can run the thing on 5.5V. They get a bit
faster as you go up from the STD 5V supply.

--

I think this thread has gone astray. The OP wanted to measure load
current, which was being obscured by totem-pole device capacitive
currents.

...Jim Thompson

Hmm... I took the OP's current-sense resistor to be for
troubleshooting, but you could be right Jim.

No worries, just move the sense resistor to the CMOS gate's Vss. Be
sure to feed the CMOS driver-part fast edges though, and/or use a
non-inverting part to avoid negative-feedback via Vss & such. (a small
Vss resistor + output resistor adds a little input hysteresis to an
ordinary, non-inverting CMOS gate.)

I suggested paralleling CMOS gates because the OP indicated he's
already using one as the driver.

Cheers,
James Arthur

Yes, and this was infact my first solution back when I did tests with
the CD4093 chip and it failed from sufficient output drive/speed

Regards

Klaus

 

[Klaus Vestergaard Kragelund]

I have a 74HCT14 hex inverter driving a NPN/PNP (BC847/BC857) totempole
stage connected to GND and 5V

Excuse me...but a PNP/NPN combo is not a "totempole." Who the hell knows
what this person is asking? I hope he communicates to himself better
than this- he will be stuck forever.

[snip]

Yeah, you're right, it's not what we normally mean by 'totem pole,'
but from this:

"The bottom PNP transistor has a current sense resistor to ground so I
can measure the current when the output falls from 5V to zero "

I took the OP to mean this:

. Vcc
. -+-
. |
. |
. |/ Q1
. +---| BC847
. | |>.
. | |
. | o----> >---o----+
. |\ | | | |
. | \ | |<' C(L) --- .-.
. --| >O--o---| 100pF --- | | R(L) = 300 ohms
. | / |\ | | |
. |/ Q2 | === '-'
. BC857 | |
. 'HCT14 .-. ===
. | |
. | | Rs
. '-'
. |
. |
. ===
. GND

Best Regards,
James Arthur

100% correct

Regards

Klaus