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Common collector amplifier and amplitude modulation

Started by Unknown April 13, 2019
Could some electronics guru here shed some light on this Could a single stage common collector amplifier exhibit amplitude modulation ?

I have the SPICE model of a simple common collector amplifier
consisting of two base bias resistors, a collector resistor and an input and a output DC blocking capacitor, as well as a source and 
a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via a RFC coil. I have tried input signal frequencies from 100 MHz 
to 750 MHz, with SPICE transient analysis. Each time, I have seen a 
output signal envelope, at the same spot on the time axis. I have
tried several common RF|microwave transistors as BFR92A, BFS17, 
BFQ67W and HFA3134, but the bproblem is always there. 

All hints and suggestions are welcome - thanks in advance.
 
On 4/12/19 11:19 PM, dakupoto@gmail.com wrote:
> Could some electronics guru here shed some light on this Could a single stage common collector amplifier exhibit amplitude modulation ? > > I have the SPICE model of a simple common collector amplifier > consisting of two base bias resistors, a collector resistor and an input and a output DC blocking capacitor, as well as a source and > a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via a RFC coil. I have tried input signal frequencies from 100 MHz > to 750 MHz, with SPICE transient analysis. Each time, I have seen a > output signal envelope, at the same spot on the time axis. I have > tried several common RF|microwave transistors as BFR92A, BFS17, > BFQ67W and HFA3134, but the bproblem is always there. > > All hints and suggestions are welcome - thanks in advance. > >
Constant-voltage base biasing is fine at audio or low RF frequencies but it's a lousy way to bias a high-frequency RF single-transistor amp. Please see e.g. <https://www.qsl.net/va3iul/Bias/Bias_Circuits_for_RF_Devices.pdf> for how to bias RF amps at those frequencies, properly...
On 4/13/19 12:12 AM, bitrex wrote:
> On 4/12/19 11:19 PM, dakupoto@gmail.com wrote: >> Could some electronics guru here shed some light on this Could a >> single stage common collector amplifier exhibit amplitude modulation ? >> >> I have the SPICE model of a simple common collector amplifier >> consisting of two base bias resistors, a collector resistor and an >> input and a output DC blocking capacitor, as well as a source and >> a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via >> a RFC coil. I have tried input signal frequencies from 100 MHz >> to 750 MHz, with SPICE transient analysis. Each time, I have seen a >> output signal envelope, at the same spot on the time axis. I have >> tried several common RF|microwave transistors as BFR92A, BFS17, >> BFQ67W and HFA3134, but the bproblem is always there. >> >> All hints and suggestions are welcome - thanks in advance. >> > > Constant-voltage base biasing is fine at audio or low RF frequencies but > it's a lousy way to bias a high-frequency RF single-transistor amp. > > Please see e.g. > <https://www.qsl.net/va3iul/Bias/Bias_Circuits_for_RF_Devices.pdf> > > for how to bias RF amps at those frequencies, properly...
That is to say it sounds like you're trying to use a variant of this circuit: <https://forum.allaboutcircuits.com/data/photos/o/0/139-1219236927-94c543bf8e11674ac5571b9e93c5795e.gif> to amplify 750MHz. the answer is fuggedaboutit.
On 2019-04-13, dakupoto@gmail.com <dakupoto@gmail.com> wrote:
> Could some electronics guru here shed some light on this Could a single stage common collector amplifier exhibit amplitude modulation ?
> I have the SPICE model of a simple common collector amplifier > consisting of two base bias resistors, a collector resistor and an input and a output DC blocking capacitor, as well as a source and > a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via a RFC coil. I have tried input signal frequencies from 100 MHz > to 750 MHz, with SPICE transient analysis.
Can you draw me a picture. that doesn't sound particularly common collector. -- When I tried casting out nines I made a hash of it.
On Fri, 12 Apr 2019 20:19:00 -0700 (PDT), dakupoto@gmail.com wrote:

>Could some electronics guru here shed some light on this Could a single stage common collector amplifier exhibit amplitude modulation ? > >I have the SPICE model of a simple common collector amplifier >consisting of two base bias resistors, a collector resistor and an input and a output DC blocking capacitor, as well as a source and >a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via a RFC coil. I have tried input signal frequencies from 100 MHz >to 750 MHz, with SPICE transient analysis. Each time, I have seen a >output signal envelope, at the same spot on the time axis. I have >tried several common RF|microwave transistors as BFR92A, BFS17, >BFQ67W and HFA3134, but the bproblem is always there. > >All hints and suggestions are welcome - thanks in advance. >
Post the Spice sim? .asc for LT Spice. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Saturday, April 13, 2019 at 9:42:06 AM UTC+5:30, bitrex wrote:
> On 4/12/19 11:19 PM, dakupoto@gmail.com wrote: > > Could some electronics guru here shed some light on this Could a single stage common collector amplifier exhibit amplitude modulation ? > > > > I have the SPICE model of a simple common collector amplifier > > consisting of two base bias resistors, a collector resistor and an input and a output DC blocking capacitor, as well as a source and > > a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via a RFC coil. I have tried input signal frequencies from 100 MHz > > to 750 MHz, with SPICE transient analysis. Each time, I have seen a > > output signal envelope, at the same spot on the time axis. I have > > tried several common RF|microwave transistors as BFR92A, BFS17, > > BFQ67W and HFA3134, but the bproblem is always there. > > > > All hints and suggestions are welcome - thanks in advance. > > > > > > Constant-voltage base biasing is fine at audio or low RF frequencies but > it's a lousy way to bias a high-frequency RF single-transistor amp. > > Please see e.g. > <https://www.qsl.net/va3iul/Bias/Bias_Circuits_for_RF_Devices.pdf> > > for how to bias RF amps at those frequencies, properly...
I have seen this paper in the past, and in fact I do have a copy on a CD. The main issue is there is no indication as to how the the passive device component values(in particular resistors) are computed. I remember in the section on current mirror based biasing the author writes something like "..with appropriate choice of R1, R2...". So how are values of R1, R2 calculated anyway ?
dakupoto@gmail.com wrote:
> Could some electronics guru here shed some light on this Could a single stage common collector amplifier exhibit amplitude modulation ? > > I have the SPICE model of a simple common collector amplifier > consisting of two base bias resistors, a collector resistor and an input and a output DC blocking capacitor, as well as a source and > a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via a RFC coil. I have tried input signal frequencies from 100 MHz > to 750 MHz, with SPICE transient analysis. Each time, I have seen a > output signal envelope, at the same spot on the time axis. I have > tried several common RF|microwave transistors as BFR92A, BFS17, > BFQ67W and HFA3134, but the bproblem is always there. > > All hints and suggestions are welcome - thanks in advance. > >
Err... isn't a "common collector amplifier" an emitter follower? And....isn't the circuit you are talking about a common emitter amplifier (base in, collector out)? --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
On Saturday, April 13, 2019 at 10:46:07 AM UTC+5:30, John Larkin wrote:
> On Fri, 12 Apr 2019 20:19:00 -0700 (PDT), dakupoto@gmail.com wrote: > > >Could some electronics guru here shed some light on this Could a single stage common collector amplifier exhibit amplitude modulation ? > > > >I have the SPICE model of a simple common collector amplifier > >consisting of two base bias resistors, a collector resistor and an input and a output DC blocking capacitor, as well as a source and > >a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via a RFC coil. I have tried input signal frequencies from 100 MHz > >to 750 MHz, with SPICE transient analysis. Each time, I have seen a > >output signal envelope, at the same spot on the time axis. I have > >tried several common RF|microwave transistors as BFR92A, BFS17, > >BFQ67W and HFA3134, but the bproblem is always there. > > > >All hints and suggestions are welcome - thanks in advance. > > > > Post the Spice sim? .asc for LT Spice. > > > > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
I use HSpice and Ngspice, both of which allow text netlists, but the circuit is so simple that I have given a description below: Vcc 15 Volt VCC -> Collector terminal 1.0uH Emitter terminal -> ground - RE -375 Ohm Vcc -> intermediate node - RB1 - 3750 Ohm Intermediate node -< ground - RB2 - 595 Ohm Intermediate node -> base terminal - RFC coil 1.0uH Both DC blocking capacitor(input, output) are 10 uF Input signal amplitude is 1.5 Volt, the frequency has been varied between 100 MHz and 750 MHz, mostly around 250 MHz
On Saturday, April 13, 2019 at 12:16:05 PM UTC+5:30, Robert Baer wrote:
> dakupoto@gmail.com wrote: > > Could some electronics guru here shed some light on this Could a single stage common collector amplifier exhibit amplitude modulation ? > > > > I have the SPICE model of a simple common collector amplifier > > consisting of two base bias resistors, a collector resistor and an input and a output DC blocking capacitor, as well as a source and > > a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via a RFC coil. I have tried input signal frequencies from 100 MHz > > to 750 MHz, with SPICE transient analysis. Each time, I have seen a > > output signal envelope, at the same spot on the time axis. I have > > tried several common RF|microwave transistors as BFR92A, BFS17, > > BFQ67W and HFA3134, but the bproblem is always there. > > > > All hints and suggestions are welcome - thanks in advance. > > > > > Err... isn't a "common collector amplifier" an emitter follower? > And....isn't the circuit you are talking about a common emitter > amplifier (base in, collector out)? > > > --- > This email has been checked for viruses by Avast antivirus software. > https://www.avast.com/antivirus
I do not have any collector resistor, and the output is taken (via a DC blocking capacitor) from the transistor emitter terminal. Please see my response to John Larkin's message.
On Saturday, 13 April 2019 08:07:54 UTC+1, daku...@gmail.com  wrote:
> On Saturday, April 13, 2019 at 12:16:05 PM UTC+5:30, Robert Baer wrote: > > dakupoto@gmail.com wrote: > > > Could some electronics guru here shed some light on this Could a single stage common collector amplifier exhibit amplitude modulation ? > > > > > > I have the SPICE model of a simple common collector amplifier > > > consisting of two base bias resistors, a collector resistor and an input and a output DC blocking capacitor, as well as a source and > > > a load resistor of 50.0 Ohm. The BJT collector is connected to Vcc via a RFC coil. I have tried input signal frequencies from 100 MHz > > > to 750 MHz, with SPICE transient analysis. Each time, I have seen a > > > output signal envelope, at the same spot on the time axis. I have > > > tried several common RF|microwave transistors as BFR92A, BFS17, > > > BFQ67W and HFA3134, but the bproblem is always there. > > > > > > All hints and suggestions are welcome - thanks in advance. > > > > > > > > Err... isn't a "common collector amplifier" an emitter follower? > > And....isn't the circuit you are talking about a common emitter > > amplifier (base in, collector out)?
> I do not have any collector resistor, and the output is taken (via a DC blocking capacitor) from the transistor emitter terminal. Please see my response to John Larkin's message.
Your RFC makes it no longer a common emitter amp. You fail to say what you actually mean by 'exhibit amplitude modulation'. NT