NPN Common Emitter Bias

[dgc]

Can an NPN transistor in a common emitter configuration be safely biased for
standing DC current WITHOUT having an emitter resistor in the circuit, i.e
emitter directly tied to ground? I'm trying to accomodate a 12 volt peak
swing on the collector with only a 12 volt supply available. Any emitter
resistor obviously lowers the peak voltage the collector can handle, while
remaining linear, so I am trying to avoid using one.

Thanks.

 

[John Popelish]

Can an NPN transistor in a common emitter configuration be safely biased for
standing DC current WITHOUT having an emitter resistor in the circuit, i.e
emitter directly tied to ground? I'm trying to accomodate a 12 volt peak
swing on the collector with only a 12 volt supply available. Any emitter
resistor obviously lowers the peak voltage the collector can handle, while
remaining linear, so I am trying to avoid using one.

Thanks.

You can use collector voltage feedback, instead.

 

[dgc]

You can use collector voltage feedback, instead.

Thanks John. I have seen collector voltage feedback modeled in a text I am
reading. However, just for my education, could a transistor be biased for
standing DC current with a direct ground connection on the emitter, without
damaging the transistor.

 

[Tim Wescott]

Can an NPN transistor in a common emitter configuration be safely biased for
standing DC current WITHOUT having an emitter resistor in the circuit, i.e
emitter directly tied to ground? I'm trying to accomodate a 12 volt peak
swing on the collector with only a 12 volt supply available. Any emitter
resistor obviously lowers the peak voltage the collector can handle, while
remaining linear, so I am trying to avoid using one.

Thanks.

It can be done, but it gets complex. "Radio Frequency Transistors --
Principals and Practical Applications" by Dye and Granberg has some
examples.

 

[John Popelish]

Thanks John. I have seen collector voltage feedback modeled in a text I am
reading. However, just for my education, could a transistor be biased for
standing DC current with a direct ground connection on the emitter, without
damaging the transistor.

If the transistor is kept at a fairly constant temperature, and the
bias is carefully set, the transistor is unlikely to be damaged, since
in a class A operation, the worst case power dissipation occurs at
zero signal. The real problem is whether the bias point will stay
centered well enough for the required signal swing without clipping.

If the temperature changes more than a few degrees, all bets are off.
Transistors are pretty good thermometers.

A simple collector voltage bias involves connecting the base divider
to the collector, instead of to the supply, with the top resistor
value dropped to account for the lower voltage across it.

 

[Pooh Bear]

Can an NPN transistor in a common emitter configuration be safely biased for
standing DC current WITHOUT having an emitter resistor in the circuit, i.e
emitter directly tied to ground?

Not easily.

I'm trying to accomodate a 12 volt peak
swing on the collector with only a 12 volt supply available. Any emitter
resistor obviously lowers the peak voltage the collector can handle, while
remaining linear, so I am trying to avoid using one.

Any objection to an IC if it's small signal ?

What's the application ?

Graham

 

[Pooh Bear]

If the transistor is kept at a fairly constant temperature, and the
bias is carefully set, the transistor is unlikely to be damaged, since
in a class A operation, the worst case power dissipation occurs at
zero signal. The real problem is whether the bias point will stay
centered well enough for the required signal swing without clipping.

If the temperature changes more than a few degrees, all bets are off.
Transistors are pretty good thermometers.

A simple collector voltage bias involves connecting the base divider
to the collector, instead of to the supply, with the top resistor
value dropped to account for the lower voltage across it.

Lower value resistors reduce the beta ( current gain ) sensitivity too.

Of course this configuration applies negative feedback - No idea if that's an
issue for the OP.

Graham

 

[Andrew Holme]

Can an NPN transistor in a common emitter configuration be safely biased for
standing DC current WITHOUT having an emitter resistor in the circuit, i.e
emitter directly tied to ground? I'm trying to accomodate a 12 volt peak
swing on the collector with only a 12 volt supply available. Any emitter
resistor obviously lowers the peak voltage the collector can handle, while
remaining linear, so I am trying to avoid using one.

Thanks.

I've seen a circuit where the NPN is biased using a PNP transistor.

From memory, it was something like an RC low-pass filter from the NPN

collector to the PNP emitter; with a potential divider setting the PNP
base voltage; and the PNP collector feeding bias to the NPN base. I
think it was a wideband RF buffer stage in a Marconi signal generator.
I'll check the service manual when I get home tonight.

 

[Fred Bloggs]

Can an NPN transistor in a common emitter configuration be safely biased for
standing DC current WITHOUT having an emitter resistor in the circuit, i.e
emitter directly tied to ground? I'm trying to accomodate a 12 volt peak
swing on the collector with only a 12 volt supply available. Any emitter
resistor obviously lowers the peak voltage the collector can handle, while
remaining linear, so I am trying to avoid using one.

This can be done any number of ways but your question cannot be answered
without knowing the frequency range of the signal to be amplified, the
required gain of the circuit, the loading, and the source
characteristics. Local feedback is not going to cut it since the
transistor gain goes to zero at cutoff and saturation.

 

[Winfield Hill]

Tim Wescott wrote...

It can be done, but it gets complex. "Radio Frequency Transistors --
Principals and Practical Applications" by Dye and Granberg has some
examples.

I don't have that book, but many wideband linear RF amplifiers I've
worked on used a current-sensing resistor in the collector RF path,
with a little servo circuit to establish the average value of the
base current. This is easy to do with just a few transistors. The
sense resistor need not drop more than 200 to 400mV.

 

[Ian Bell]

Can an NPN transistor in a common emitter configuration be safely biased
for standing DC current WITHOUT having an emitter resistor in the circuit,
i.e
emitter directly tied to ground? I'm trying to accomodate a 12 volt peak
swing on the collector with only a 12 volt supply available. Any emitter
resistor obviously lowers the peak voltage the collector can handle, while
remaining linear, so I am trying to avoid using one.

Thanks.

Yes, but you need feedback of a different sort. My ascii art is crap so I
will describe one type of circuit. Collector resistor to Vcc, emitter to
gnd. Resistor from collector to base. Resistor from base to gnd. Collector
volts is essentially vbe multiplied by the collector/base base/gnd resistor
ratio. This sets both ac and dc gain. You can separate ac gain by slitting
the collector/base resistor into two equal parts and decoupling the
junction to ground.

HTH

Ian

 

[Jim Thompson]

Yes, but you need feedback of a different sort. My ascii art is crap so I
will describe one type of circuit. Collector resistor to Vcc, emitter to
gnd. Resistor from collector to base. Resistor from base to gnd. Collector
volts is essentially vbe multiplied by the collector/base base/gnd resistor
ratio. This sets both ac and dc gain. You can separate ac gain by slitting
the collector/base resistor into two equal parts and decoupling the
junction to ground.

HTH

Ian

AND: TC of output DC bias point = -2mV/°C * DCGAIN

...Jim Thompson

 

[Andrew Holme]

I've seen a circuit where the NPN is biased using a PNP transistor.
collector to the PNP emitter; with a potential divider setting the PNP
base voltage; and the PNP collector feeding bias to the NPN base. I
think it was a wideband RF buffer stage in a Marconi signal generator.

They used lots of variations: always with a resistor between PNP collector
and NPN base; often with a large capacitor from PNP collector to ground;
only sometimes with a capacitor from PNP emitter to ground; and sometimes
with a small NPN emitter resistor. None are anything like rail-to-rail
output. Sometimes there's a diode in series with the potential divider -
presumably for temperature compensation. I also found an example where they
used an NPN instead of the PNP, but enough said.

This is from a 500-900 MHz first LO drive circuit:


.------------------------o------- +10V
| |
| .-.
| | | 100-ohm
| | |
.-. '-'
| | |
| | .-------o-------o
'-' | | |
| | | |
| |< --- |
o------| --- 1n C|
| |\ | C| L
.-. | | C|
| | | === |
| | .-. GND |
'-' | | o-------- out
| | | 1k5 |
=== '-' |
GND | |
| |/
in --------------o-------------|
|>
|
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

 

[Ian Bell]

AND: TC of output DC bias point = -2mV/°C * DCGAIN

Quite right, and probably the reason it is rarely used. In simple
applications where temperature range is limited it may not be a problem.
With a 12V supply and a 6V collector voltage the temp co is about
20mV/degree which over a 20 degree range is just 400mV.

Ian

 

[John Larkin]

Quite right, and probably the reason it is rarely used. In simple
applications where temperature range is limited it may not be a problem.
With a 12V supply and a 6V collector voltage the temp co is about
20mV/degree which over a 20 degree range is just 400mV.

Ian

Well, just run the low side of the bottom resistor to a negative
voltage. That's a great way to bias gaasfets, too.

John

 

[Ian Bell]

Well, just run the low side of the bottom resistor to a negative
voltage. That's a great way to bias gaasfets, too.

John

I suspect if the OP had a negative voltage available he would want the
output to swing all the way down to it ;-)

Ian

 

[Winfield Hill]

Andrew Holme wrote...

They used lots of variations: always with a resistor between PNP collector
and NPN base; often with a large capacitor from PNP collector to ground;
only sometimes with a capacitor from PNP emitter to ground; and sometimes
with a small NPN emitter resistor. None are anything like rail-to-rail
output. Sometimes there's a diode in series with the potential divider -
presumably for temperature compensation. I also found an example where they
used an NPN instead of the PNP, but enough said.

This is from a 500-900 MHz first LO drive circuit:

.------------------------o------- +10V
| |
| .-.
| | | 100-ohm
| | |
.-. '-'
| | |
| | .-------o-------o
'-' | | |
| | | |
| |< --- |
o------| --- 1n C|
| |\ | C| L
.-. | | C|
| | | === |
| | .-. GND |
'-' | | o-------- out
| | | 1k5 |
=== '-' |
GND | |
| |/
in --------------o-------------|
|>
|
|
===
GND

The circuit has three problems, all easily fixed. First,
the bypass cap should go across the sense resistor, etc.,
because the goal is to get a constant base-bias current
to the RF transistor, unaffected by supply fluctuations,
RF currents, etc. Second, we don't want the servo-loop
gain too high at high frequencies, so some PNP emitter
resistance is in order. Third, we'd like to reduce the
wasted voltage drop across the sense resistor, yet avoid
tempco issues with the PNP, so we need a tempco-matching
transistor. Here's the result of these changes.

.. .---------------------+----+------- +Vcc
.. | | |
.. R1 _|_ Rs
.. 250mV --- 200mV
.. | | |
.. v\| .--/\/\--+----+
.. |---, | 50mV |
.. /| | |/v |
.. +-----+----| C|
.. | |\ C| L
.. R2. | C|
.. 5mA | |
.. etc | |
.. | | +-------- out
.. GND | |
.. | |/
.. RF IN------+-----------|
.. |\v
.. |
.. ===
.. GND

 

[[email protected]]

Andrew Holme wrote...

The circuit has three problems, all easily fixed. First,
the bypass cap should go across the sense resistor, etc.,
because the goal is to get a constant base-bias current
to the RF transistor, unaffected by supply fluctuations,
RF currents, etc. Second, we don't want the servo-loop
gain too high at high frequencies, so some PNP emitter
resistance is in order. Third, we'd like to reduce the
wasted voltage drop across the sense resistor, yet avoid
tempco issues with the PNP, so we need a tempco-matching
transistor. Here's the result of these changes.

. .---------------------+----+------- +Vcc
. | | |
. R1 _|_ Rs
. 250mV --- 200mV
. | | |
. v\| Q3 .--/\/\--+----+
. |---, | 50mV |
. /| | |/v |
. +-----+----| C|
. | Q2 |\ C| L
. R2. | C|
. 5mA | |
. etc | |
. | | +-------- out
. GND | |
. | Q1 |/
. RF IN------+-----------|
. |\v
. |
. ===
. GND

Gee, I thought I'd invented this scheme, used in a 920MHz RF power
amp way back when ;-) But heck, I was a noob, hacking together some
stuff for the project of the day.

Two quibbles: the inductor must be bypassed, else the added series
resistance can spoil its 'Q', and, more importantly, you'll get
undesired feedback around Q1, collector-to-base through the
current-sensing transistor Q2.

Secondly, Andrew's resistor in the collector of biasing/current
sensing transistor Q2 is a good idea, to avoid loading the base of RF
amplifier tranny Q1.

Cheers,
James Arthur

 

[[email protected]]

Winfield Hill wrote:

The circuit has three problems, all easily fixed. [...]

Here's a version that makes sure the inductor is bypassed at RF
frequencies, yet, per Win, avoids AM modulation of the bias from power
supply noise:

.. .---------------------+----------+------- +Vcc
.. | C1 | |
.. R1 100n _|_ Rs
.. 300mV --- 200mV
.. | R3 | R4 |
.. v\| .--/\/\--+--/\/\----+--------.
.. |---, | 50mV 50mV | |
.. /| | |/v | --- C2
.. +-----+----| C| --- 1n
.. | Q2 |\ C| L1 |
.. R2. | C| ===
.. 5mA .-. | GND
.. etc | | R5 |
.. | | | 1k +------------ out
.. GND '-' |
.. | Q1 |/
.. RF IN------+-----------------|
.. |\v
.. |
.. ===
.. GND

Regards,
James Arthur

 

[Winfield Hill]

[email protected] wrote...C2 = 1nF ??