What's wrong with this amplifier?

P

Paul Burridge

Guest
Hi all,

I picked up this simple class C amp for 30Mhz on the 'net. It claims
to provide 30dBm gain over the stage for a total of 1W out. I'd hoped
to be able to tweak the values for 40Mhz, but can't even get the
quoted gain out of it at its design frequency, let alone anything
else. My Spice simulations so far have only yielded a small and not
worthwhile voltage gain, I obtained all the parts in advance, but am
reluctant to make a board and build it since the sim is so
disappointing. Can anyone verify the simulation or say if the circuit
might still work in practice?


VCC=15V
|
.-.
| |
39R| | +------+
'-' | |
| | C|33uH RFC
+----+-----+ C|
| C|
--- + 1.5uH 1.2uH 10R
1u --- | ___ ___ || ___
| 150pF +-------+UUU--+-UUU---||---|___|----OUT
| IN || |/ | ||
| ----||-+-+-| 2N5109 | 1000pF
| || | |> |
| | + +-----++
| | | | |
=== --- | | |
GND D1 ^ | --- ---
| | --- --- 39pF
| | | |
(1N914) | | | |
+----+-------+------+--------------+
|
15pF (variable) ===
GND

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

--

"I believe history will be kind to me, since I intend
to write it." - Winston Churchill
 
"Paul Burridge" <pb@osiris1.notthisbit.co.uk> wrote in message
news:crlblvslcgati9a76nvajkis57g1vfq46n@4ax.com...
Hi all,

I picked up this simple class C amp for 30Mhz on the 'net. It claims
to provide 30dBm gain over the stage for a total of 1W out. I'd hoped
to be able to tweak the values for 40Mhz, but can't even get the
quoted gain out of it at its design frequency, let alone anything
else. My Spice simulations so far have only yielded a small and not
worthwhile voltage gain, I obtained all the parts in advance, but am
reluctant to make a board and build it since the sim is so
disappointing. Can anyone verify the simulation or say if the circuit
might still work in practice?


VCC=15V
|
.-.
| |
39R| | +------+
'-' | |
| | C|33uH RFC
+----+-----+ C|
| C|
--- + 1.5uH 1.2uH 10R
1u --- | ___ ___ || ___
| 150pF +-------+UUU--+-UUU---||---|___|----OUT
| IN || |/ | ||
| ----||-+-+-| 2N5109 | 1000pF
| || | |> |
| | + +-----++
| | | | |
=== --- | | |
GND D1 ^ | --- ---
| | --- --- 39pF
| | | |
(1N914) | | | |
+----+-------+------+--------------+
|
15pF (variable) ===
GND

created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de
Click to expand...
You didnt mention the driving voltage or power. With no bias, an input
coupling capacitor, and Class C operation, it's going to need at least 1.3
volts peak-to-peak at a low, under 200 ohms I'd guess, input impedance
before you get even one microwatt out of it. Probably will run much better
with at least 3 volts p-p.

And SPICE won't tell you, but you'll probably need a small resistor or
ferrite bead in the base lead to prevent getting a visit from some peeved
Military communications folks (these tend to oscillate right in their
~200MHz radio allocation).
 
Paul Burridge <pb@osiris1.notthisbit.co.uk> wrote in message
news:crlblvslcgati9a76nvajkis57g1vfq46n@4ax.com...
Hi all,

I picked up this simple class C amp for 30Mhz on the 'net. It claims
to provide 30dBm gain over the stage for a total of 1W out. I'd hoped
to be able to tweak the values for 40Mhz, but can't even get the
quoted gain out of it at its design frequency, let alone anything
else. My Spice simulations so far have only yielded a small and not
worthwhile voltage gain, I obtained all the parts in advance, but am
reluctant to make a board and build it since the sim is so
disappointing. Can anyone verify the simulation or say if the circuit
might still work in practice?


VCC=15V
|
.-.
| |
39R| | +------+
'-' | |
| | C|33uH RFC
+----+-----+ C|
| C|
--- + 1.5uH 1.2uH 10R
1u --- | ___ ___ || ___
| 150pF +-------+UUU--+-UUU---||---|___|----OUT
| IN || |/ | ||
| ----||-+-+-| 2N5109 | 1000pF
| || | |> |
| | + +-----++
| | | | |
=== --- | | |
GND D1 ^ | --- ---
| | --- --- 39pF
| | | |
(1N914) | | | |
+----+-------+------+--------------+
|
15pF (variable) ===
GND

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

--

"I believe history will be kind to me, since I intend
to write it." - Winston Churchill
Click to expand...
If ever I see "Dbm's" or "Db's" (or suchlike) quoted for an isolated
circuit block, I smell trouble lurking and move on. It's a purposely vague,
wishy-washy way of avoiding having to quote any useful measurements.
According to this means of characterisation, our complete test gear needs
would be satisfied by a Bird Thru-line watt-meter.
The author should really have specified the AC input and output voltages and
the source and load resistances and ignored the pretentious Dbm thing. My
'scope measures voltages, he's obviously got one of those special Dbm scaled
versions that only snotty RF engineers are allowed to use.
Back on subject, the circuit is no good for 40MHz. The LP Tee network on the
output is only good for a span of maybe 7-30MHz (tops) and would need
redesigning for 40MHz.
Best I could get using a 50ohm load and various transistors was 12Vpp out
for a 2Vpp (at 15ma rms from a let's call it a 50ohm source resistance).
Nothing like the quoted "30Dbm" output (=7Vrms=21Vpp). The circuit would
work better, setup as a class A common emitter amp.
Power in is 2Vpp/2.8 X 15ma = 0.01 Watts
Output current is (12V/2.8) / 50ohms = 0.085Arms.
So Output power is 0.085 X 0.085 X 50ohms = 0.36Watts
Total amplifier Power gain is therefore 0.36 / 0.01 = 36X
And in damned Db's, 10 X Log10(36) = +15Db's
Damned Db's (wrt 1mW in 50ohm) gain at the output is 0.36Watts / 0.001Watts
=360X
so output power DB's is 10 X Log10(360) = +25Dbm.
"+15Db", "+25Dbm" "+30Dbm", Confusing innit? ... That's what the Db pushers
usually intend.
regards
john
PS. At these 'near DC' frequencies, a good circuit should not be sensitive
to a change of input signal diode.
 
Paul Burridge wrote:
Hi all,

I picked up this simple class C amp for 30Mhz on the 'net. It claims
to provide 30dBm gain over the stage for a total of 1W out.
Click to expand...
There is no such thing as "dBm gain." Coincidentally, 30 dBm is the
same thing as 1 W. Do you mean 30 dB of gain? In that case the input
(driving) power to get 1 W would, of course, be 0 dBm. Nothing is
stated here about source or load impedances, although 50 Ohms might be a
good assumption to start with. That resistor in the output seems a bit
of a waste, no? I mean, I've heard of de-Q'ing loads but.... Same goes
for the collector resistor, although maybe you don't about efficiency.

I'd hoped
to be able to tweak the values for 40Mhz, but can't even get the
quoted gain out of it at its design frequency, let alone anything
else. My Spice simulations so far have only yielded a small and not
worthwhile voltage gain, I obtained all the parts in advance, but am
reluctant to make a board and build it since the sim is so
disappointing. Can anyone verify the simulation or say if the circuit
might still work in practice?

VCC=15V
|
.-.
| |
39R| | +------+
'-' | |
| | C|33uH RFC
+----+-----+ C|
| C|
--- + 1.5uH 1.2uH 10R
1u --- | ___ ___ || ___
| 150pF +-------+UUU--+-UUU---||---|___|----OUT
| IN || |/ | ||
| ----||-+-+-| 2N5109 | 1000pF
| || | |> |
| | + +-----++
| | | | |
=== --- | | |
GND D1 ^ | --- ---
| | --- --- 39pF
| | | |
(1N914) | | | |
+----+-------+------+--------------+
|
15pF (variable) ===
GND

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

--

"I believe history will be kind to me, since I intend
to write it." - Winston Churchill
Click to expand...
 
"Paul Burridge" <pb@osiris1.notthisbit.co.uk> wrote in message
news:1tsblv0rvdtv6i6pmu1voqlhk1ntsjijbg@4ax.com...


And SPICE won't tell you, but you'll probably need a small resistor or
ferrite bead in the base lead to prevent getting a visit from some peeved
Military communications folks (these tend to oscillate right in their
~200MHz radio allocation).

Not at the power levels under discussion.
Click to expand...

Do you mean it won't oscillate at these power levels, or even if it does,
you won't get a visit?

Remember than 50mw can go really far line-of-sight. Hams have gone 100's
of miles at these power levels, with coathanger antennas and lousy
superregen receivers.

Oh, and it would be very chancy to trust SPICE to tell you if it's going to
oscillate. Many oscillations depend on unquantified and unmodelable fringe
elements. The only sure way is to build the circuit and put a widebansd
spectrum analyzer on it.
 
Hi Paul:

I simulated your amp in LT Spice and also came up with poor performance.
The gain is best at 21.7 MHz, but not worthwhile. As I get more time, I'll
make some changes to determine if reasonable performance can be had (via
simulation).
 
On Thu, 4 Sep 2003 04:14:01 +0100, "John Jardine"
<john@jjdesigns.fsnet.co.uk> wrote:

If ever I see "Dbm's" or "Db's" (or suchlike) quoted for an isolated
circuit block, I smell trouble lurking and move on. It's a purposely vague,
wishy-washy way of avoiding having to quote any useful measurements.
According to this means of characterisation, our complete test gear needs
would be satisfied by a Bird Thru-line watt-meter
Click to expand...
Thanks, John. I'm always grateful for your input in this subject area,
seeing as you're an RF-orientated sort of a chap.

The author should really have specified the AC input and output voltages and
the source and load resistances and ignored the pretentious Dbm thing. My
'scope measures voltages, he's obviously got one of those special Dbm scaled
versions that only snotty RF engineers are allowed to use.
Click to expand...
My scope only does voltage as well. But since P=V^2/R it's not hard to
convert from one to t'other.

Back on subject, the circuit is no good for 40MHz. The LP Tee network on the
output is only good for a span of maybe 7-30MHz (tops) and would need
redesigning for 40MHz.
Click to expand...
He's actually provided component values for three frequency bands:
30Mhz, 100Mhz and 200Mhz all using the same circuit (but with a
different tranny for the 200Mhz variant). Is it possible to post a
..pdf file to abse? You could see for yourself where he's coming from.

Best I could get using a 50ohm load and various transistors was 12Vpp out
for a 2Vpp (at 15ma rms from a let's call it a 50ohm source resistance).
Click to expand...
My findings exactly also!

Nothing like the quoted "30Dbm" output (=7Vrms=21Vpp). The circuit would
work better, setup as a class A common emitter amp.
Power in is 2Vpp/2.8 X 15ma = 0.01 Watts
Output current is (12V/2.8) / 50ohms = 0.085Arms.
So Output power is 0.085 X 0.085 X 50ohms = 0.36Watts
Total amplifier Power gain is therefore 0.36 / 0.01 = 36X
And in damned Db's, 10 X Log10(36) = +15Db's
Damned Db's (wrt 1mW in 50ohm) gain at the output is 0.36Watts / 0.001Watts
=360X
so output power DB's is 10 X Log10(360) = +25Dbm.
"+15Db", "+25Dbm" "+30Dbm", Confusing innit? ... That's what the Db pushers
usually intend.
Click to expand...
That's a tad unkind, John. dBm/dBw and so forth are frequently used
terms for describing the gain sequence of a number of stages leading
up to a final PA as you will undoubtedly know. Highly prevalent
terminology by manufacturers and designers alike.
Having said all that, though. I must admit I'm *much* more at home
with your preferred way of describing things.

PS. At these 'near DC' frequencies, a good circuit should not be sensitive
to a change of input signal diode.
Click to expand...
Agreed. Good point and one I hadn't considered! If you try simulating
it and make really very minor changes to the diode model; you'll be
amazed how much it screws overall performance up. :-(
--

"I believe history will be kind to me, since I intend
to write it." - Winston Churchill
 
On Wed, 03 Sep 2003 22:04:58 -0700, gwhite <gwhite@ti.com> wrote:

There is no such thing as "dBm gain." Coincidentally, 30 dBm is the
same thing as 1 W. Do you mean 30 dB of gain?
Click to expand...
Quite correct. I had misgivings about this immediately upon hitting
the 'send' button. Yes, the designer quotes that for a 13dBm input,
the stage will give 30dBm out. I hope that clarifies things a bit.



--

"I believe history will be kind to me, since I intend
to write it." - Winston Churchill
 
On Thu, 4 Sep 2003 16:19:22 -0400, "Charles Schuler"
<charlesschuler@comcast.net> wrote:

Hi Paul:

I simulated your amp in LT Spice and also came up with poor performance.
The gain is best at 21.7 MHz, but not worthwhile. As I get more time, I'll
make some changes to determine if reasonable performance can be had (via
simulation).
Click to expand...
Thanks, Charles. I'd be very interested to see what results you get.
Remember the overall aim is to be able to tweak the component values
to get a useful output at 40Mhz. I use LT Spice as well so your
findings will be of particular interest to me.

--

"I believe history will be kind to me, since I intend
to write it." - Winston Churchill
 
Thanks, Charles. I'd be very interested to see what results you get.
Remember the overall aim is to be able to tweak the component values
to get a useful output at 40Mhz. I use LT Spice as well so your
findings will be of particular interest to me.
Click to expand...
No luck, Paul. I can't get reasonable voltages and currents. I reached a
point where the ac analysis started showing a little gain at 40 MHz, but the
transient analysis does not yield reasonable voltages and currents. I also
tried a different circuit configuration ... one that I know works in real
life and does not have an input diode. No joy there either. I used what is
shown below for the transistor but eliminated the subcircuit section since
this should not be required at 40 MHz (although, it might add some accuracy
if the simulation would ever justify using it). I am fairly new to LT
Spice, so maybe I'm doing something wrong.

..SUBCKT QN5109 1 2 3

LC 1 4 0.875E-9

LB 2 6 1.590E-9

LE 5 3 2.650E-9

CC 4 3 1.410E-12

CB 4 6 0.561E-12

Q1 4 6 5 QR33

..MODEL QR33 NPN ( BF=44 VAF=160 VAR=16.0 RC=0.69

+RB=1.57 RE=2.75 IKF=0.28E+00 ISE=0.36E-13 TF=0.111E-09

+TR=0.80E-08 ITF=0.82E-01 VTF=0.66E+01 CJC=2.758E-12

+CJE=1.822E-12 XTI=3.0 NE=1.5 ISC=0.12E-13 EG=1.11

+XTB=1.5 BR=1.14 VJC=0.75 VJE=0.75 IS=0.40E-14

+MJC=0.33 MJE=0.33 XTF=4.0 IKR=0.28E+00 KF=0.1E-14

+NC=1.7 FC=0.50 RBM=1.1 IRB=0.40E-01 XCJC=0.5 )

..ENDS
 
Charles Schuler wrote:
Thanks, Charles. I'd be very interested to see what results you get.
Remember the overall aim is to be able to tweak the component values
to get a useful output at 40Mhz. I use LT Spice as well so your
findings will be of particular interest to me.

No luck, Paul. I can't get reasonable voltages and currents. I
reached a point where the ac analysis started showing a little gain
at 40 MHz,
Click to expand...
Oh dear... Oh dear...Oh dear...an AC analysis, right off the bat, is
absolutely useless for a class c amp. One should not even contemplate
it. Everything is bloody well turned off!. All your'll get is signal
bleed through.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
 
"Kevin Aylward" <kevin@anasoft.co.uk> wrote in message
news:Mt26b.7550$Ci1.1087624@newsfep2-win.server.ntli.net...
Charles Schuler wrote:
Thanks, Charles. I'd be very interested to see what results you get.
Remember the overall aim is to be able to tweak the component values
to get a useful output at 40Mhz. I use LT Spice as well so your
findings will be of particular interest to me.

No luck, Paul. I can't get reasonable voltages and currents. I
reached a point where the ac analysis started showing a little gain
at 40 MHz,

Oh dear... Oh dear...Oh dear...an AC analysis, right off the bat, is
absolutely useless for a class c amp. One should not even contemplate
it. Everything is bloody well turned off!. All your'll get is signal
bleed through.
Click to expand...
Oh, really? Why is it then that one gets to specify the amplitude of the
source that will be used during the small signal AC Analyses? I thought
that was the very purpose of this parameter?
 
Charles,

Oh dear... Oh dear...Oh dear...an AC analysis, right off the bat, is
absolutely useless for a class c amp. One should not even contemplate
it. Everything is bloody well turned off!. All your'll get is signal
bleed through.

Oh, really? Why is it then that one gets to specify the amplitude of the
source that will be used during the small signal AC Analyses? I thought
that was the very purpose of this parameter?
Click to expand...
The only purpose of that parameter is to let you reference the
signal level to something other than zero dB. Whether it is 1uV
or 1TeraVolt doesn't make any difference once you reference
all the signal levels to whatever value you used. The .ac analysis
is a linear analysis of the circuit linearized about the operating
point with no AC signals.

--Mike
 
"Charles Schuler" <charlesschuler@comcast.net> wrote in message
news:vIedneZHE_JtTMWiU-KYgw@comcast.com...

Oh, really? Why is it then that one gets to specify the amplitude of the
source that will be used during the small signal AC Analyses? I thought
that was the very purpose of this parameter?
Click to expand...

SPICE's user interface is not psychic, clairvoyant, pre-cognitive, or even
intelligent.

It has no way of knowing whether you've set up the proper DC biasing
conditions for a AC signal to ride thru on.

This is a Class C amplifier, which is as far from "small-signal" operation
as one can get.


In this circuit, nothing happens with a small AC input.

The signal has to be on the order of 1.3 volts p-p before anything will
happen.

At that level the diode across the input will start to conduct, thereby
back-charging the input capacitor (WHICH HAD BETTER COME FROM A SOURCE WITH
LOW DC IMPEDANCE!) With the input capacitor charged up a bit, then the
positive half cycle can transfer some of that charge to the base of the
transistor.
 
Charles,

The only purpose of that parameter is to let you reference the
signal level to something other than zero dB. Whether it is 1uV
or 1TeraVolt doesn't make any difference once you reference
all the signal levels to whatever value you used. The .ac analysis
is a linear analysis of the circuit linearized about the operating
point with no AC signals.

OK, let's see if I understand this. The AC analysis will not change (the
gain shown in dB) unless the circuit goes into clipping?
Click to expand...
The .ac analysis is a linear analysis. If you double the input
signal amplitude the output amplitude doubles. This is true over
the dynamic range of the computer's floating point math, not the
circuit's. You will not see clipping by stepping the size of
the ac source's ac amplitude. To see clipping, or any other non-
linear effect, you can do a .tran analysis with the ac sources
replaced with sine function sources.

--Mike
 
Oh boy, now I'm really confused. I set up a voltage divider with equal
resistors and get -6 dB only when the right-hand side source amplitude is
set to 1 V. I guess I'm really dense but still don't see the exact purpose
of this second amplitude parameter.
 
On Fri, 5 Sep 2003 15:26:38 -0400, "Charles Schuler"
<charlesschuler@comcast.net> wrote:

"Mike Engelhardt" <pmte@concentric.net> wrote in message
news:bjak5g$rla@dispatch.concentric.net...
Charles,

Oh dear... Oh dear...Oh dear...an AC analysis, right off the bat, is
absolutely useless for a class c amp. One should not even contemplate
it. Everything is bloody well turned off!. All your'll get is signal
bleed through.

Oh, really? Why is it then that one gets to specify the amplitude of
the
source that will be used during the small signal AC Analyses? I thought
that was the very purpose of this parameter?

The only purpose of that parameter is to let you reference the
signal level to something other than zero dB. Whether it is 1uV
or 1TeraVolt doesn't make any difference once you reference
all the signal levels to whatever value you used. The .ac analysis
is a linear analysis of the circuit linearized about the operating
point with no AC signals.

OK, let's see if I understand this. The AC analysis will not change (the
gain shown in dB) unless the circuit goes into clipping?
Click to expand...
AC analysis uses a small-signal-linearized model, so there is no such
thing as clipping.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
On Fri, 5 Sep 2003 16:13:46 -0400, "Charles Schuler"
<charlesschuler@comcast.net> wrote:

Oh boy, now I'm really confused. I set up a voltage divider with equal
resistors and get -6 dB only when the right-hand side source amplitude is
set to 1 V. I guess I'm really dense but still don't see the exact purpose
of this second amplitude parameter.
Click to expand...
Which simulator?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
"Jim Thompson" <Jim-T@golana-will-get-you.com> wrote in message
news:abshlvoi7vcdh7s32qcg4t61d453sr11s9@4ax.com...
On Fri, 5 Sep 2003 16:13:46 -0400, "Charles Schuler"
charlesschuler@comcast.net> wrote:

Oh boy, now I'm really confused. I set up a voltage divider with equal
resistors and get -6 dB only when the right-hand side source amplitude is
set to 1 V. I guess I'm really dense but still don't see the exact
purpose
of this second amplitude parameter.


Which simulator?
Click to expand...
LT Spice. Help me Jim! (I'm stuck in loop)
 
On Fri, 5 Sep 2003 16:32:07 -0400, "Charles Schuler"
<charlesschuler@comcast.net> wrote:

"Jim Thompson" <Jim-T@golana-will-get-you.com> wrote in message
news:abshlvoi7vcdh7s32qcg4t61d453sr11s9@4ax.com...
On Fri, 5 Sep 2003 16:13:46 -0400, "Charles Schuler"
charlesschuler@comcast.net> wrote:

Oh boy, now I'm really confused. I set up a voltage divider with equal
resistors and get -6 dB only when the right-hand side source amplitude is
set to 1 V. I guess I'm really dense but still don't see the exact
purpose
of this second amplitude parameter.


Which simulator?

LT Spice. Help me Jim! (I'm stuck in loop)
Click to expand...
PSpice displays (in Probe) dBv, that is dB relative to a volt, so if
you change the reference by 20dB the output will scale accordingly.

However the difference (in dB) between input and output will always be
6dB (in your resistor case). Likewise an amplifier's GAIN will not
change with change in input reference level.

I suspect that LTSpice follows that same convention.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
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