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Definition of Colpitts Oscillator

S

sjcma

Jan 1, 1970
0
Hi,

I'm hoping someone can enlighten me on the definition(s) of a Colpitts
oscillator. I've seen many circuits diagrams that show a Colpitts
oscillator, but they don't always look the same.

Assuming an NPN device, some show the capacitive feedback from collector
to emitter, some show the capacitor feedback from emitter to base, and
some show the tap between the 2 tank capacitors grounded. I've seen
circuits that are common base, common emitter, and common collector all
claming to be Colpitts.

What's the common link between all these circuits besides tapping the
tank in between two capacitors and connecting it somewhere (sometimes,
even ground!).

If someone feels like typing a lot, perhaps a quick explanation of the
advantages of each topology would be nice :)

Thanks in advance.

sjcma
 
B

BFoelsch

Jan 1, 1970
0
sjcma said:
Hi,

I'm hoping someone can enlighten me on the definition(s) of a Colpitts
oscillator. I've seen many circuits diagrams that show a Colpitts
oscillator, but they don't always look the same.

Assuming an NPN device, some show the capacitive feedback from collector
to emitter, some show the capacitor feedback from emitter to base, and
some show the tap between the 2 tank capacitors grounded. I've seen
circuits that are common base, common emitter, and common collector all
claming to be Colpitts.

What's the common link between all these circuits besides tapping the tank
in between two capacitors and connecting it somewhere (sometimes, even
ground!).

Nothing. The "tapped tank capacitance," as you call it , is the definition
of a Colpitts oscillator. If you take the same circuit, but tap the
inductor, you have a Hartley oscillator.

Both oscillators may be common emitter, base, or collector. Oscillators like
that are identified by the AC circuit, where the circuit happens to be
grounded or how the power is applied is irrelevant.
If someone feels like typing a lot, perhaps a quick explanation of the
advantages of each topology would be nice :)

Maybe in the morning, I'm pooped right now.
 
T

Tim Wescott

Jan 1, 1970
0
BFoelsch said:
Nothing. The "tapped tank capacitance," as you call it , is the definition
of a Colpitts oscillator. If you take the same circuit, but tap the
inductor, you have a Hartley oscillator.

Both oscillators may be common emitter, base, or collector. Oscillators like
that are identified by the AC circuit, where the circuit happens to be
grounded or how the power is applied is irrelevant.




Maybe in the morning, I'm pooped right now.
The best illustration of this I've seen is to draw the circuit without
bias networks, like this:


.-------o----------.
| | |
| --- |/
| --- .---|
C| | | |>
C| o----' |
C| | |
| --- |
| --- |
| | |
| | |
'-------o----------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Now ground whichever node you want, add power and bias, and Presto! you
have a working amplifier circuit! (if you want an oscillator you'll
have to try to make a stable amplifier).
 
R

Rich Grise

Jan 1, 1970
0
Hi,

I'm hoping someone can enlighten me on the definition(s) of a Colpitts
oscillator. I've seen many circuits diagrams that show a Colpitts
oscillator, but they don't always look the same.

Assuming an NPN device, some show the capacitive feedback from collector
to emitter, some show the capacitor feedback from emitter to base, and
some show the tap between the 2 tank capacitors grounded. I've seen
circuits that are common base, common emitter, and common collector all
claming to be Colpitts.

What's the common link between all these circuits besides tapping the
tank in between two capacitors and connecting it somewhere (sometimes,
even ground!).

As far as I know, there isn't. The way I learned it, the definition
of a Colpitts is a parallel resonant circuit with a tapped capacitor,
with signal ground at the junction of the caps, so the ends are 180
degrees out of phase, so it's a natural feedback path. Or something
like that. One end to the output and the other to the input, basically.
And the Hartley is the same thing with the coil and caps swapped. ;-)

The rest is window dressing. :)
If someone feels like typing a lot, perhaps a quick explanation of the
advantages of each topology would be nice :)

All I can say to this is, of course, "It depends." :)

But I'd bet credits to navy beans that a google search will turn up
reading material that can keep you occupied for awhile. :)

Cheers!
Rich
 
K

Kevin Aylward

Jan 1, 1970
0
sjcma said:
Hi,

I'm hoping someone can enlighten me on the definition(s) of a Colpitts
oscillator. I've seen many circuits diagrams that show a Colpitts
oscillator, but they don't always look the same.

Assuming an NPN device, some show the capacitive feedback from
collector to emitter, some show the capacitor feedback from emitter
to base, and some show the tap between the 2 tank capacitors
grounded. I've seen circuits that are common base, common emitter,
and common collector all claming to be Colpitts.

What's the common link between all these circuits besides tapping the
tank in between two capacitors and connecting it somewhere (sometimes,
even ground!).

If someone feels like typing a lot, perhaps a quick explanation of the
advantages of each topology would be nice :)

Thanks in advance.

Well, I disagree with the other definitions in this thread on the taped
cap, especially the one by Tim with no cap directly from collector to
emitter. Here's the real deal.

Consider 3 impedances on a transister. Zcb, Zbe Zce.

The load on the collecter is Zce || (Zcb + Zbe).

The loop gain is therefore:

Av = gm.(Zce || (Zcb + Zbe)).Zbe/(Zbe + Zcb)

Setting this to one, one finds that the only too solutions are where
Zbe, Zce are capacitice with Zcb inductive or Zbe, Zce are inductive
with Zcb capacitive.

That is, a Colpits oscillator is when, topological, there are caps
across base emitter and collector emitter, with an inductance from base
to collector (Colpits), or where the caps and inducters are swaped
(Hartly).

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
 
T

Tim Wescott

Jan 1, 1970
0
Tim said:
The best illustration of this I've seen is to draw the circuit without
bias networks, like this:


.-------o----------.
| | |
| --- |/
| --- .---|
C| | | |>
C| o----' |
C| | |
| --- |
| --- |
| | |
| | |
'-------o----------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Now ground whichever node you want, add power and bias, and Presto! you
have a working amplifier circuit! (if you want an oscillator you'll
have to try to make a stable amplifier).

@#$%! See Kevin Aylward's comments -- I got my transistor in there
wrong. All other comments apply.


.------o------.
| | |
| --- |
| --- |
C| | ---
C| o-----v \--.
C| | |
| --- |
| --- |
| | |
'------o----------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
 
T

Tim Wescott

Jan 1, 1970
0
Kevin said:
Well, I disagree with the other definitions in this thread on the taped
cap, especially the one by Tim with no cap directly from collector to
emitter. Here's the real deal.

Consider 3 impedances on a transister. Zcb, Zbe Zce.

The load on the collecter is Zce || (Zcb + Zbe).

The loop gain is therefore:

Av = gm.(Zce || (Zcb + Zbe)).Zbe/(Zbe + Zcb)

Setting this to one, one finds that the only too solutions are where
Zbe, Zce are capacitice with Zcb inductive or Zbe, Zce are inductive
with Zcb capacitive.

That is, a Colpits oscillator is when, topological, there are caps
across base emitter and collector emitter, with an inductance from base
to collector (Colpits), or where the caps and inducters are swaped
(Hartly).
Argh. Yes, I got the transistor in there wrong -- I was concentrating
on the nifty notion that you draw the circuit free of any entanglements
from bias networks, then you ground whatever point is most convenient
for you.

Oddly enough I almost never do that in practice -- its only when I have
an audience that I screw up in such a stupid way.
 
K

Kevin Aylward

Jan 1, 1970
0
Tim said:
Argh. Yes, I got the transistor in there wrong -- I was concentrating
on the nifty notion that you draw the circuit free of any
entanglements from bias networks, then you ground whatever point is
most convenient for you.

Oddly enough I almost never do that in practice -- its only when I
have an audience that I screw up in such a stupid way.

I thought I made a mistake once, but that was an error.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
 
R

Rich The Philosophizer

Jan 1, 1970
0
Oddly enough I almost never do that in practice -- its only when I have
an audience that I screw up in such a stupid way.

Don't worry, Tim. _I_ saw the one that got away. ;-)
 
S

sjcma

Jan 1, 1970
0
Rich said:
On Sat, 13 Nov 2004 21:23:13 -0500, sjcma wrote:


As far as I know, there isn't. The way I learned it, the definition
of a Colpitts is a parallel resonant circuit with a tapped capacitor,
with signal ground at the junction of the caps, so the ends are 180
degrees out of phase, so it's a natural feedback path. Or something
like that. One end to the output and the other to the input, basically.
And the Hartley is the same thing with the coil and caps swapped. ;-)

The rest is window dressing. :)

Hi Rich,

I've seen circuits like that described as being a Colpitts oscillator.
But I've also seen the exact same circuit being described as a Pierce
oscillator, where an inverting amplifier (could be a digital inverter)
is attached to a parallel-C, series L or crystal, and parallel-C before
feeding back to the input of the inverting amplifier.

So is it a Colpitts or Pierce? Or is the Pierce name strictly for xtal
oscillators?

Stan
 
S

sjcma

Jan 1, 1970
0
Kevin said:
Well, I disagree with the other definitions in this thread on the taped
cap, especially the one by Tim with no cap directly from collector to
emitter. Here's the real deal.

Consider 3 impedances on a transister. Zcb, Zbe Zce.

The load on the collecter is Zce || (Zcb + Zbe).

The loop gain is therefore:

Av = gm.(Zce || (Zcb + Zbe)).Zbe/(Zbe + Zcb)

Setting this to one, one finds that the only too solutions are where
Zbe, Zce are capacitice with Zcb inductive or Zbe, Zce are inductive
with Zcb capacitive.

That is, a Colpits oscillator is when, topological, there are caps
across base emitter and collector emitter, with an inductance from base
to collector (Colpits), or where the caps and inducters are swaped
(Hartly).

Kevin Aylward

Hi Kevin,

Thanks for a brilliantly clear explanation.

So how should one go about choosing which of the three terminals of the
transistor to ground? What are the pluses and minuses of each of the
three options?

Stan
 
R

Rich Grise

Jan 1, 1970
0
Hi Rich,

I've seen circuits like that described as being a Colpitts oscillator.
But I've also seen the exact same circuit being described as a Pierce
oscillator, where an inverting amplifier (could be a digital inverter)
is attached to a parallel-C, series L or crystal, and parallel-C before
feeding back to the input of the inverting amplifier.

So is it a Colpitts or Pierce? Or is the Pierce name strictly for xtal
oscillators?
I'd have to look that up, but from memory, the difference would be
the tapped capacitor. The frequency-determining circuit in the Colpitts
is the parallel resonant ciruit formed by the inductor L in paralled
with a capacitance C which is the total capacitance of the series
capacitors.

The way I understand it, in the Pierce, the frequency-determining
element stands on its own merit, and the cap on the other end of
the tank, while providing feedback, doesn't participate in
determining the frequency of oscillation. I'm sure you can look
this stuff up - and while you're at it, look up Clapp and Armstrong.
:)

Here's a leg up:
http://www.google.com/search?q=hartley+colpitts+pierce&btnG=Google+Search

HTH!
Rich
 
A

Active8

Jan 1, 1970
0
Kevin Aylward wrote:

Argh. Yes, I got the transistor in there wrong -- I was concentrating
on the nifty notion that you draw the circuit free of any entanglements
from bias networks, then you ground whatever point is most convenient
for you.

Oddly enough I almost never do that in practice -- its only when I have
an audience that I screw up in such a stupid way.
@#$%! See Kevin Aylward's comments -- I got my transistor in there
wrong. All other comments apply.

.------o------.
| | |
| --- |
| --- |
C| | ---
C| o-----v \--.
C| | |
| --- |
| --- |
| | |
'------o----------'

What about - I've seen this, but it's an n-fet, not an npn. The c-e
cap isn't part of the tank, per se, but it's a bypass - ?

+---------+
| |
| Bypass |
|/ ---
+----------+------| ---
| | |> |
| --- | |
| --- | |
C| | | |
C| +--------+---- out |
C| | |
| --- |
| --- |
| | |
+----------+------------------+-- GND, IIRC

created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de
 
A

Active8

Jan 1, 1970
0
So how should one go about choosing which of the three terminals of the
transistor to ground? What are the pluses and minuses of each of the
three options?

My guesses:

CE - big swing moderate output Z.

CC - low Z out for driving a buffer. Osc isn't loaded or Miller
f*cked by the load.

CB - keeps the swing from exacerbating the Miller / Early effects.

'twas a good qweshtion :)
 
R

Rich Grise

Jan 1, 1970
0
What about - I've seen this, but it's an n-fet, not an npn. The c-e
cap isn't part of the tank, per se, but it's a bypass - ?
Yes - the cap is a short for RF, so from the RF's POV, it's the same
circuit.
+---------+
| |
| Bypass |
|/ ---
+----------+------| ---
| | |> |
| --- | |
| --- | |
C| | | |
C| +--------+---- out |
C| | |
| --- |
| --- |
| | |
+----------+------------------+-- GND, IIRC

created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

Cheers!
Rich
 
A

Active8

Jan 1, 1970
0
Yes - the cap is a short for RF, so from the RF's POV, it's the same
circuit.


Cheers!
Rich

Then there are those osc tranny specs/notes where they hardly add
any components - it runs off junction caps. I think the output cap
is reflected to the input as inductance in significant enough
amounts to affect the tank.

It's one of the amps that do.
 
T

Tim Wescott

Jan 1, 1970
0
Active8 said:
Then there are those osc tranny specs/notes where they hardly add
any components - it runs off junction caps. I think the output cap
is reflected to the input as inductance in significant enough
amounts to affect the tank.

It's one of the amps that do.

There are any number of ways to make an oscillator using the parasitic
components in the transistor. Other than overtone crystal oscillators
which use the parasitic C-B capacitor to form a Hartley oscillator
(actually a tuned-base tuned-collector oscillator) between the crystal
on the base and the tank circuit on the collector I haven't actually
used any of that sort.
 
R

Rich The Philosophizer

Jan 1, 1970
0
There are any number of ways to make an oscillator using the parasitic
components in the transistor. Other than overtone crystal oscillators
which use the parasitic C-B capacitor to form a Hartley oscillator
(actually a tuned-base tuned-collector oscillator) between the crystal
on the base and the tank circuit on the collector I haven't actually
used any of that sort.

On one of my first PCs, I had a 2-serial/parallel board, but the
parallel port on the board used some Chinese lame-chip, that died
if you looked at it wrong. This wasn't a problem for me, except
that it also hosted the oscillator for the two UARTS. Since this
was back in the days when I was young and had energy, I built
a daughterboard with TTL that duplicated the circuit right out
of the IBM Tech Ref, except plugged in in place of the Chinese
chip. The oscillator I did separately on the same board, and it
consisted of one transistor (2n2222 equiv.), the crystal, a cap,
2 resistors, and a choke for the collector that I made by winding
about 25 turns of about #36 wire on a 3/4" piece of toothpick
with cut-off resistor leads glued into little holes I poked in
its ends. Oh, and Vee-Vcc clamp diodes.

I think it was a Pierce or Clapp oscillator - about the simplest
one-tranny xtal osc you can make.

I never blew up the LPT port after that, and the UARTS worked!

Cheers!
Rich
 
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