Transistorized "cascode" RF amp?
 

Back from good old tube days, cascoded triode RF preamps were good for high
gain, stability and low noise.

Anything out there in transistorised version, or there is there better stuff
available? Looking mainly for preamps on HF to be used with low gain antennas,
like small loops or beverages.

Yuri, K3BU.us

Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually
uses it in the transmitter ... even with a passive mixer you don't need a
lot of gain on 40!

See http://www.qsl.net/k8iqy

...

"Yuri Blanarovich" wrote in message
...
Back from good old tube days, cascoded triode RF preamps were good for
high
gain, stability and low noise.

Anything out there in transistorised version, or there is there better
stuff
available? Looking mainly for preamps on HF to be used with low gain
antennas,
like small loops or beverages.

Yuri, K3BU.us

Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually
uses it in the transmitter ... even with a passive mixer you don't need a
lot of gain on 40!

See http://www.qsl.net/k8iqy

...

"Yuri Blanarovich" wrote in message
...
Back from good old tube days, cascoded triode RF preamps were good for
high
gain, stability and low noise.

Anything out there in transistorised version, or there is there better
stuff
available? Looking mainly for preamps on HF to be used with low gain
antennas,
like small loops or beverages.

Yuri, K3BU.us

"Yuri Blanarovich" wrote in message
...
Back from good old tube days, cascoded triode RF preamps were good for
high
gain, stability and low noise.

Anything out there in transistorised version, or there is there better
stuff
available? Looking mainly for preamps on HF to be used with low gain
antennas,
like small loops or beverages.

Yuri, K3BU.us

Back in the 'good old tube days' it was difficult to get below a NF of 3 db
on 2 meters. Now it is easy to get way below that with a good FET. For HF
almost any RF device now will take you very low in NF with good gain.

There are versions of the cascode that use FETs for HF and above.

"Yuri Blanarovich" wrote in message
...
Back from good old tube days, cascoded triode RF preamps were good for
high
gain, stability and low noise.

Anything out there in transistorised version, or there is there better
stuff
available? Looking mainly for preamps on HF to be used with low gain
antennas,
like small loops or beverages.

Yuri, K3BU.us

Back in the 'good old tube days' it was difficult to get below a NF of 3 db
on 2 meters. Now it is easy to get way below that with a good FET. For HF
almost any RF device now will take you very low in NF with good gain.

There are versions of the cascode that use FETs for HF and above.

Yuri Blanarovich ) writes:
Back from good old tube days, cascoded triode RF preamps were good for high
gain, stability and low noise.

Anything out there in transistorised version, or there is there better stuff
available? Looking mainly for preamps on HF to be used with low gain antennas,
like small loops or beverages.

Yuri, K3BU.us

You definitely saw them in the early days. The one that comes immediately to
mind was a cheap preamp described in Ham Radio in the early seventies,
and was available as a cheap kit from Hamtronics.

I think there were some shown with bipolars, but I can't dredge up
any specific memories at the moment.

It was obviously a transitional thing, since you're right, one doesn't
see solid state cascodes very often. I have no idea if they were
used decades ago because it worked around limitations of early solid
state devices, or if it just seemed to be the thing to do since they
had been common in tube circuits.

The author of that Ham Radio article (actually, there were two, and he
used cascode FET amplifiers in various projects described later), I think
his name was Jerry Voigt, had grumlbed about the then relatively new
dual-gate MOSFETs, but in a followup letter he admitted than a dual-gate
MOSFET was basically a cascode device.

If there was an advantage to be using cascode circuits today, then
you'd be seeing them.

A lot has changed. Bipolars came in, and they couldn't handle strong
signals well. The JFET came along, and it was seen as the device
to use for best performance. The MOSFET came along, and again there
was a switch. Sort of simultaneously with MOSFETs, but not really adopted
till later, bipolars started being capable of low noise and strong signal
handling. MOSFETs are now rare for receiving applications. Gasfets came
along and they seem to be be the thing for low noise RF amplification.

Michael VE2BVW

Yuri Blanarovich ) writes:
Back from good old tube days, cascoded triode RF preamps were good for high
gain, stability and low noise.

Anything out there in transistorised version, or there is there better stuff
available? Looking mainly for preamps on HF to be used with low gain antennas,
like small loops or beverages.

Yuri, K3BU.us

You definitely saw them in the early days. The one that comes immediately to
mind was a cheap preamp described in Ham Radio in the early seventies,
and was available as a cheap kit from Hamtronics.

I think there were some shown with bipolars, but I can't dredge up
any specific memories at the moment.

It was obviously a transitional thing, since you're right, one doesn't
see solid state cascodes very often. I have no idea if they were
used decades ago because it worked around limitations of early solid
state devices, or if it just seemed to be the thing to do since they
had been common in tube circuits.

The author of that Ham Radio article (actually, there were two, and he
used cascode FET amplifiers in various projects described later), I think
his name was Jerry Voigt, had grumlbed about the then relatively new
dual-gate MOSFETs, but in a followup letter he admitted than a dual-gate
MOSFET was basically a cascode device.

If there was an advantage to be using cascode circuits today, then
you'd be seeing them.

A lot has changed. Bipolars came in, and they couldn't handle strong
signals well. The JFET came along, and it was seen as the device
to use for best performance. The MOSFET came along, and again there
was a switch. Sort of simultaneously with MOSFETs, but not really adopted
till later, bipolars started being capable of low noise and strong signal
handling. MOSFETs are now rare for receiving applications. Gasfets came
along and they seem to be be the thing for low noise RF amplification.

Michael VE2BVW

xpyttl wrote:
Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually
uses it in the transmitter ... even with a passive mixer you don't need a
lot of gain on 40!

See http://www.qsl.net/k8iqy

..
There are two ways of looking at cascade, the devices are in series for
dc and ac, or only ac. The above link shows the latter. I had an idea
for a hybrid cascade amp of the first kind. The input would be a jfet
which sorta becomes the emitter resistor for a bipolar transistor as the
second stage of the cascade amp. The bipolar is run in grounded base
(for ac, normal bias network and base bypassed to ground with a cap).
Would give the advantages of high input impedance of the fet and high
gain from the bipolar.

xpyttl wrote:
Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually
uses it in the transmitter ... even with a passive mixer you don't need a
lot of gain on 40!

See http://www.qsl.net/k8iqy

..
There are two ways of looking at cascade, the devices are in series for
dc and ac, or only ac. The above link shows the latter. I had an idea
for a hybrid cascade amp of the first kind. The input would be a jfet
which sorta becomes the emitter resistor for a bipolar transistor as the
second stage of the cascade amp. The bipolar is run in grounded base
(for ac, normal bias network and base bypassed to ground with a cap).
Would give the advantages of high input impedance of the fet and high
gain from the bipolar.

Ken Scharf ) writes:
xpyttl wrote:
Jim Kortge, K8IQY, uses a cascode amp in his famous 2N2/40. He actually
uses it in the transmitter ... even with a passive mixer you don't need a
lot of gain on 40!

See http://www.qsl.net/k8iqy

..
There are two ways of looking at cascade, the devices are in series for
dc and ac, or only ac. The above link shows the latter. I had an idea
for a hybrid cascade amp of the first kind. The input would be a jfet
which sorta becomes the emitter resistor for a bipolar transistor as the
second stage of the cascade amp. The bipolar is run in grounded base
(for ac, normal bias network and base bypassed to ground with a cap).
Would give the advantages of high input impedance of the fet and high
gain from the bipolar.

It's been done. There was an article in Ham Radio magazine in 1970 or
1971 about a six meter converter. Just as you described, a common source
JFET with a bipolar transistor as the upper element. Used a Vackar oscillator
to make it tuneable. It might be this one:
Six-meter converter, improved K1BQT 50 Aug 70
The date is about when I remember it, though I don't remember K1BQT being
the author.

Michael VE2BVW