I'm having trouble understanding how the typical shunt feedback
networks used in RF (solid state) amps work. I'm looking at the 1993
ARRL Handbook. Typical common base broadband amp. For the shunt
feedback (from collector to base) they have two resistors: 560 ohms in
series with 3300 ohms. The 3300 ohm is bypassed by a .01 uf cap.

So far so good. But then the text explains that because you have
rising gain characteristics when the frequency drops you need something
to reduce gain at lower frequencies. That's why the negative feedback
helps.

Here's where I'm having trouble: "As the operating frequency is
decreased the negative feedback increases becasue the network feedback
reactance becomes lower." Huh? Wouldn't that network's reactance
INCREASE as frequency is lowered? The only part of it with reactance
is the .01 cap, correct?

Help! 73!

Bill M0HBR N2CQR CU2JL
http://www.qsl.net/n2cqr

wrote:
I'm having trouble understanding how the typical shunt feedback
networks used in RF (solid state) amps work. I'm looking at the 1993
ARRL Handbook. Typical common base broadband amp. For the shunt
feedback (from collector to base) they have two resistors: 560 ohms in
series with 3300 ohms. The 3300 ohm is bypassed by a .01 uf cap.

So far so good. But then the text explains that because you have
rising gain characteristics when the frequency drops you need something
to reduce gain at lower frequencies. That's why the negative feedback
helps.

Here's where I'm having trouble: "As the operating frequency is
decreased the negative feedback increases becasue the network feedback
reactance becomes lower." Huh? Wouldn't that network's reactance
INCREASE as frequency is lowered? The only part of it with reactance
is the .01 cap, correct?

I think the capacitor is large enough that you can consider it a short
circuit for all frequencies (above 1/(2*pi*3300*.01*10^6)=4.8kHz). So
the author has confused himself about that. The feedback passes
through an effectively a constant impedance of 560 ohms. The 3300 ohm
resistor and capacitor are just there to set the DC operating point.
As the low frequency gain rises, the feedback current increases as the
output voltage tries to increase, but this has nothing to do with a
change in the feedback path impedance.

On 18 Oct 2005 22:58:55 -0700, wrote:

I'm having trouble understanding how the typical shunt feedback
networks used in RF (solid state) amps work. I'm looking at the 1993
ARRL Handbook. Typical common base broadband amp. For the shunt
feedback (from collector to base) they have two resistors: 560 ohms in
series with 3300 ohms. The 3300 ohm is bypassed by a .01 uf cap.

So far so good. But then the text explains that because you have
rising gain characteristics when the frequency drops you need something
to reduce gain at lower frequencies. That's why the negative feedback
helps.

Here's where I'm having trouble: "As the operating frequency is
decreased the negative feedback increases becasue the network feedback
reactance becomes lower." Huh? Wouldn't that network's reactance
INCREASE as frequency is lowered? The only part of it with reactance
is the .01 cap, correct?

In the 1994 RAH the same reference is in section 4 figure 53C.

There are TWO feedback networks. One in the emitter circuit
and the Collector Base feedback. Ther is also the transformer
that will limit low frequecy response as a secondary action
which also limits Collector feedback as well.

However, they are refering to the RC pair found on the leg of the
emitter. For that case the statement would be correct.

Typically those amps (unless higher power) the emitter has a bias
resistor. That resistor is bypassed with a RC of .1 or .01uF and
series R of a few ohms (typically less than 20 ohms). That C is
fairly small for low RF and audio to effectively bypass the emitter
circuit. so at say DC and audio the Emitter resistance is 110ohms.
At some greater frequency the capacitor looks like a very low
reactance and the emitter resistance is effectively 10 ohms at RF.

That emitter network allows setting DCbias (operating point) and also
the AC feedback at the Emitter Base leg. Since the capacitor
has greater reactance at low frequencies the feedback is greater.

The RAH has only a very terse description and lacks design detail.
If you look at Solid State Design for the Radio Amateur (ARRL press)
and/or Exprimental Methods in RF Design (ARRL press) you will
have a far more detailed explanation of how that amplifier works and
how to calculate values for design.

Hope that helps.

Allison

wrote in message
oups.com...
I'm having trouble understanding how the typical shunt feedback
networks used in RF (solid state) amps work. I'm looking at the 1993
ARRL Handbook. Typical common base broadband amp. For the shunt
feedback (from collector to base) they have two resistors: 560 ohms in
series with 3300 ohms. The 3300 ohm is bypassed by a .01 uf cap.

So far so good. But then the text explains that because you have
rising gain characteristics when the frequency drops you need something
to reduce gain at lower frequencies. That's why the negative feedback
helps.

Here's where I'm having trouble: "As the operating frequency is
decreased the negative feedback increases becasue the network feedback
reactance becomes lower." Huh? Wouldn't that network's reactance
INCREASE as frequency is lowered? The only part of it with reactance
is the .01 cap, correct?

Help! 73!

Bill M0HBR N2CQR CU2JL
http://www.qsl.net/n2cqr


In the 70's I worked on 150 MHz PA's and we used what we called a
"banana". It was called that because once one got fried and afterward it
looked like a cooked banana. It was a orange drop (dipped) cap (I think
mylar), value forgotten - somewhere in the .001 - .01 range, with two 1/4
watt resistors in series, one at each end that held it up over the power
transistor.
Though I don't remember if they were ever measured, the rationale was
that these caps had considerable inductance at 150 MHz. and were thus an
"open" there. Down in the 1-20 MHz range, where the regen (regeneration -
oscillation) occurred, they were a "short". Also remember that the
transistor impedances are in the .1-1 ohm range for power devices. I think
we also may have put small, 50 ohm, beads on the resistor leads. If the
bead exploded, you knew you hadn't sloved the regen problem because there
was considerable energy at the regen frequency. and thay became good loads.

For regen there are the "dancing faintlies" and the "christmas tree" types.
(:-)

Forget not those parasitics in the components.
73, Steve, K,9.D;C'i

On Wed, 19 Oct 2005 13:16:45 -0500, "Steve Nosko"
wrote:


wrote in message
roups.com...
I'm having trouble understanding how the typical shunt feedback
networks used in RF (solid state) amps work. I'm looking at the 1993
ARRL Handbook. Typical common base broadband amp. For the shunt
feedback (from collector to base) they have two resistors: 560 ohms in
series with 3300 ohms. The 3300 ohm is bypassed by a .01 uf cap.

So far so good. But then the text explains that because you have
rising gain characteristics when the frequency drops you need something
to reduce gain at lower frequencies. That's why the negative feedback
helps.

Here's where I'm having trouble: "As the operating frequency is
decreased the negative feedback increases becasue the network feedback
reactance becomes lower." Huh? Wouldn't that network's reactance
INCREASE as frequency is lowered? The only part of it with reactance
is the .01 cap, correct?

Help! 73!

Bill M0HBR N2CQR CU2JL
http://www.qsl.net/n2cqr


In the 70's I worked on 150 MHz PA's and we used what we called a
"banana". It was called that because once one got fried and afterward it
looked like a cooked banana. It was a orange drop (dipped) cap (I think
mylar), value forgotten - somewhere in the .001 - .01 range, with two 1/4
watt resistors in series, one at each end that held it up over the power
transistor.
Though I don't remember if they were ever measured, the rationale was
that these caps had considerable inductance at 150 MHz. and were thus an
"open" there. Down in the 1-20 MHz range, where the regen (regeneration -
oscillation) occurred, they were a "short". Also remember that the
transistor impedances are in the .1-1 ohm range for power devices. I think
we also may have put small, 50 ohm, beads on the resistor leads. If the
bead exploded, you knew you hadn't sloved the regen problem because there
was considerable energy at the regen frequency. and thay became good loads.

For regen there are the "dancing faintlies" and the "christmas tree" types.
(:-)

Forget not those parasitics in the components.
73, Steve, K,9.D;C'i

Having tamed a few solid state power amps I can appreciate that.
However this is not a similar case.

The amplifier in question is of the lower power (under 50mW) type
commonly used for wide band amplifiers and/or RF amps at low levels.
The question really stems from a far too brief description of said amp
in the handbook.

Allison
KB1GMX

Allison: Thanks.

Obviously you are right. But I see the same kind of confusing language
in other editions of the handbook. In the 1980 edition (otherwise one
of my favorites) they have (pg 6-19) a collector-base feedback network
with just a cap and a resistor in series. Text reads: "C1 and R3
provide negative feedback which increases progressively as the
frequency is lowered." It calls for a cap of values between 220 pf and
..00015 for hf band amps, IN SERIES with a resistor of from 51 to 5600
ohms.

Is this just a case of some confusing language that has kind of worked
its way in the handbook DNA and is passed down from generation to
generation? Or is there really something about these cap and resistor
in series networks that cause them to increase the amount of feedback
as freq drops?

73 Bill M0HBR N2CQR CU2JL
http://www.qsl.net/n2cqr

On 19 Oct 2005 22:29:03 -0700, wrote:

Allison: Thanks.

Obviously you are right. But I see the same kind of confusing language
in other editions of the handbook. In the 1980 edition (otherwise one
of my favorites) they have (pg 6-19) a collector-base feedback network
with just a cap and a resistor in series. Text reads: "C1 and R3
provide negative feedback which increases progressively as the
frequency is lowered." It calls for a cap of values between 220 pf and
.00015 for hf band amps, IN SERIES with a resistor of from 51 to 5600
ohms.

Is this just a case of some confusing language that has kind of worked
its way in the handbook DNA and is passed down from generation to
generation? Or is there really something about these cap and resistor
in series networks that cause them to increase the amount of feedback
as freq drops?

73 Bill M0HBR N2CQR CU2JL
http://www.qsl.net/n2cqr

That reference is definately an error. The only way that would be
expected to occur is when the C is sufficiently large enough to be
useful at lower frequencies. Often enough it was.

Amplifiers of that type and time use transistors of say FT 150mhz
and a betacutoff of 1 to 3 mhz. What happen is as you go up in
frequency from 1 to say 30 mhz the effective AC beta goes from
about 100 down to 5. The amplifier gain also has that same
downward curve. It's that rising gain when progressing down in
frequency your working against with the described network.
So when that allowed for and the series C and R it's is possible
to say that as the feedback increases as the gain increases.
Even then without putting boundaries on the values and using
a real (working) circuit it's not a good explanation and borders
on error.

The "yabut" of amplifiers (that one being a class C poweramp)
is that EVERYthing around the transistor interacts due to the low
impedences and high currents. So by observation that circuit
may actually work somewhat as described but likely there are
onter factors at work that are not well explained there.

Examples of this (same circuit P6-19) that can cause grief. The
bypass on the collector circuit needs to be effective at audio through
RF and yet the schematic only shows one cap. The likely circuit
when built would not be stable.

I think again that explanations suffers from trying to be too brief
and leaving out detail. When examined without full context
appears to be an error can still have validity. The RAH is a good
text, lots of information and relatively few errors. However it's
only one source and even they cite other sources for greater
detail. Failure to take advantage of those citations and delve
deeper can lead to believeing in lore rather than studied
consideration.


Allison

The 1994 ARRL handbook, page 4-23, Fig 53C shows the feedback amplifier that
you describe. However, it is a common-emitter circuit, not a common-base
circuit. The 3300 ohm resistor returns to +12 V DC and provides DC base
current for the 2N5109. The 0.01 uF bypass makes the 560 ohm resistor the
main source of this feedback at radio frequencies.

There are two kinds of feedback in the circuit. One is the 560 ohms in the
base circuit. The author calls this "negative feedback". I call this
"voltage feedback". This feedback does not change very much over the HF
region.

The other feedback (he calls it "degenerative feedback") is a 10 ohm
resistor in the emitter in series with a 100 resistor which is shunted by
0.01uF. I call this "current feedback". This feedback increases at low
radio frequency because the impedance from emitter to ground increases at
low radio frequencies. This is the feedback that the text is referring to
in the text and it is correct. This kind of feedback increases at low
frequency. If the 0.01 uF were replaced by a 1.0 uF this increase in
feedback would be a lot less at low radio frequencies.

The author, probably DeMaw, got his terminology slightly mixed up but he is
referring to the emitter to ground current feedback, not the collector to
base voltage feedback.

If you have a copy of the 2004 Handbook, chapter 17 has a sidebar discussion
of negative feedback that is interesting. Later editions may have deleted
it.

Bill W0IYH

wrote in message
oups.com...
I'm having trouble understanding how the typical shunt feedback
networks used in RF (solid state) amps work. I'm looking at the 1993
ARRL Handbook. Typical common base broadband amp. For the shunt
feedback (from collector to base) they have two resistors: 560 ohms in
series with 3300 ohms. The 3300 ohm is bypassed by a .01 uf cap.

So far so good. But then the text explains that because you have
rising gain characteristics when the frequency drops you need something
to reduce gain at lower frequencies. That's why the negative feedback
helps.

Here's where I'm having trouble: "As the operating frequency is
decreased the negative feedback increases becasue the network feedback
reactance becomes lower." Huh? Wouldn't that network's reactance
INCREASE as frequency is lowered? The only part of it with reactance
is the .01 cap, correct?

Help! 73!

Bill M0HBR N2CQR CU2JL
http://www.qsl.net/n2cqr

wrote:



I'm having trouble understanding how the typical shunt feedback
networks used in RF (solid state) amps work. I'm looking at the 1993
ARRL Handbook. Typical common base broadband amp. For the shunt
feedback (from collector to base) they have two resistors: 560 ohms in
series with 3300 ohms. The 3300 ohm is bypassed by a .01 uf cap.

So far so good. But then the text explains that because you have
rising gain characteristics when the frequency drops you need something
to reduce gain at lower frequencies. That's why the negative feedback
helps.

Here's where I'm having trouble: "As the operating frequency is
decreased the negative feedback increases becasue the network feedback
reactance becomes lower." Huh? Wouldn't that network's reactance
INCREASE as frequency is lowered? The only part of it with reactance
is the .01 cap, correct?

Help! 73!

Bill M0HBR N2CQR CU2JL
http://www.qsl.net/n2cqr

I think to actually figure this out you will need to write the gain transfer
equation for the amplifier. Many of the replies here are talking about an
RF amplifier but are analyzing the feedback at DC.

Remember that the output of a transistor is NOT in phase with the input. As
the base voltage goes UP the collector voltage goes DOWN. (think of a
transistor used as a switch - when the base is biased off the no current
flows and the collector is at power supply potential - when the switch is
biased on then current flows and the collector is driven toward the
potential of the emitter -- i.e. the collector voltage goes down) For RF
the actual phase difference between input and output is dependent upon the
input and output impedances of the transistor as well as the gain transfer
characteristic (things like transit times of the current carriers and
junctions widths and all sorts of stuff figure in here). *THEN* you have to
consider the phase contribution of the RC network in the collector to base
network. At some RF frequency the collector is 180deg out of phase with the
input so a direct feedback link from the collector to the base would be
*negative*. It is quite possible that the phase relationships of this
particular amplifier are such that the negative feedback increases as the
frequency goes down.

You would just have to write the equations and see where they take you.

tim ab0wr

Thanks to all who responded to my question.

I went back and took a closer look at other editions of the Handbook,
and at Solid State Design for the Radio Amateur (SSDRA).

The Handbooks all seem to suggest that the shunt feedback networks
using a resistor and a cap in series will somehow result in more
negative feedback at lower frequencies. Given that capacitive
reactance varies inversely with freq, I still can't see how this
happens. Tim's message seems to provide one possible explanation, but
I'm kind of surprised that I haven't seen mention of this in the ham
literature.

When SSDRA discusses shunt feedback, (Chapter 8, page 188) they have a
resistor, a blocking cap, and an inductor all in series between the
collector and the base in a common emmiter amp. "The inductor has the
effect of decreasing the feedback at high frequencies, while the 470
ohm resistor is the dominant element at low frequencies." That's easy
to see. But without the inductor (no inductor in the Handbook
presentations) it is hard to see how this works.

Thanks again to all,

73 Bill M0HBR CU2JL N2CQR
http://www.qsl.net/n2cqr

tim gorman wrote:
wrote:



I'm having trouble understanding how the typical shunt feedback
networks used in RF (solid state) amps work. I'm looking at the 1993
ARRL Handbook. Typical common base broadband amp. For the shunt
feedback (from collector to base) they have two resistors: 560 ohms in
series with 3300 ohms. The 3300 ohm is bypassed by a .01 uf cap.

So far so good. But then the text explains that because you have
rising gain characteristics when the frequency drops you need something
to reduce gain at lower frequencies. That's why the negative feedback
helps.

Here's where I'm having trouble: "As the operating frequency is
decreased the negative feedback increases becasue the network feedback
reactance becomes lower." Huh? Wouldn't that network's reactance
INCREASE as frequency is lowered? The only part of it with reactance
is the .01 cap, correct?

Help! 73!

Bill M0HBR N2CQR CU2JL
http://www.qsl.net/n2cqr

I think to actually figure this out you will need to write the gain transfer
equation for the amplifier. Many of the replies here are talking about an
RF amplifier but are analyzing the feedback at DC.

Remember that the output of a transistor is NOT in phase with the input. As
the base voltage goes UP the collector voltage goes DOWN. (think of a
transistor used as a switch - when the base is biased off the no current
flows and the collector is at power supply potential - when the switch is
biased on then current flows and the collector is driven toward the
potential of the emitter -- i.e. the collector voltage goes down) For RF
the actual phase difference between input and output is dependent upon the
input and output impedances of the transistor as well as the gain transfer
characteristic (things like transit times of the current carriers and
junctions widths and all sorts of stuff figure in here). *THEN* you have to
consider the phase contribution of the RC network in the collector to base
network. At some RF frequency the collector is 180deg out of phase with the
input so a direct feedback link from the collector to the base would be
*negative*. It is quite possible that the phase relationships of this
particular amplifier are such that the negative feedback increases as the
frequency goes down.

You would just have to write the equations and see where they take you.

tim ab0wr