working circuit
Hi Fred,
as BJT´s have a much better behaviour (noise! and thermal
stability) in a CCS than JFET´s (or worse MosFET´s) I chose them.
Of course is anybody free to swap them if desired.
)
There is plenty of gain since this topology is nearly the same as
the famous X-Amps EXCEPT XA!
The openloop gain is more like 100k/100 since the output bleeder
resistor is 100k.
In fact its much lower in reality.
This unit is designed as a replacement for gain stages NOT as
a standalone working circuit, just like an OpAmp. You need to
add some external circuit to fit this in an existing design.
As the current thru the output devices is approximately 5mA
you can easily calculate the output driving capacity.
As there is no amp-specific feedback applied the
outputimpedance is fairly high. this impedance depends on the
applied feedback to form a closed loop.
Concerning DC-offset its up to the users choice:
either putting some capacitance to the output
or apply some sort of DC-servo circuit.
thanks
Uli
Fred Dieckmann said:
Hi Fred,
as BJT´s have a much better behaviour (noise! and thermal
stability) in a CCS than JFET´s (or worse MosFET´s) I chose them.
Of course is anybody free to swap them if desired.
)There is plenty of gain since this topology is nearly the same as
the famous X-Amps EXCEPT XA!
The openloop gain is more like 100k/100 since the output bleeder
resistor is 100k.
In fact its much lower in reality.
This unit is designed as a replacement for gain stages NOT as
a standalone working circuit, just like an OpAmp. You need to
add some external circuit to fit this in an existing design.
As the current thru the output devices is approximately 5mA
you can easily calculate the output driving capacity.
As there is no amp-specific feedback applied the
outputimpedance is fairly high. this impedance depends on the
applied feedback to form a closed loop.
Concerning DC-offset its up to the users choice:
either putting some capacitance to the output
or apply some sort of DC-servo circuit.
thanks
Uli
much better behaviour (on my part for a change)
" BJT´s have a much better behaviour (noise! and thermal
stability) in a CCS than JFET´s"
Not really in actually circuit. Thermal stabilty is largely a function of the degeneration and stability of the reference. The same goes for noise. I have never seen a BJT curret source without a voltage
reference and emitter resistor. Direct comparison with Jfet current source using Ids and the gate to source resistor with no reference is really impossible. The jfet usually has a higher output impedance for the same amount of degeneration and output impedance is the usally the most important parameter for a CCS.
"There is plenty of gain since this topology is nearly the same as
the famous X-Amps EXCEPT XA!"
The Xamps use a mosfet follower output stage and the open loop
largely independent of the load impedance. The gain of your preamp is a direct function of the load impededance. 100k amp input impedance will give a 50K ohm load to the common source
gain stage. 10K input impedances (now common for inverting power amps) will really decrease the open loop gain. This is a real issue driving the X Amps or inverting gain clones people on the forum are building. Cable capacitance is a another real influence on this this topology as well. Put about 500 pF to ground on the output and see what happens. I don't know of I would describe the the topology as having plenty off gain driving a 10K load.
I mention this because I did a look of looking at the influence of loads and input impedances on the circuit with Spice modeling. Things get very interesting when you drive a single ended input amp from just one output of the preamp circuit. Simple circuits are never sthat simple when you examine them in terms of real world loads and I wonder if there some limitations of this topology in driving typical amplifier loads and being driven single ended from a 10k to 100K pot typicall used in a preamp circuit. Thanks for your response.
Fred
" BJT´s have a much better behaviour (noise! and thermal
stability) in a CCS than JFET´s"
Not really in actually circuit. Thermal stabilty is largely a function of the degeneration and stability of the reference. The same goes for noise. I have never seen a BJT curret source without a voltage
reference and emitter resistor. Direct comparison with Jfet current source using Ids and the gate to source resistor with no reference is really impossible. The jfet usually has a higher output impedance for the same amount of degeneration and output impedance is the usally the most important parameter for a CCS.
"There is plenty of gain since this topology is nearly the same as
the famous X-Amps EXCEPT XA!"
The Xamps use a mosfet follower output stage and the open loop
largely independent of the load impedance. The gain of your preamp is a direct function of the load impededance. 100k amp input impedance will give a 50K ohm load to the common source
gain stage. 10K input impedances (now common for inverting power amps) will really decrease the open loop gain. This is a real issue driving the X Amps or inverting gain clones people on the forum are building. Cable capacitance is a another real influence on this this topology as well. Put about 500 pF to ground on the output and see what happens. I don't know of I would describe the the topology as having plenty off gain driving a 10K load.
I mention this because I did a look of looking at the influence of loads and input impedances on the circuit with Spice modeling. Things get very interesting when you drive a single ended input amp from just one output of the preamp circuit. Simple circuits are never sthat simple when you examine them in terms of real world loads and I wonder if there some limitations of this topology in driving typical amplifier loads and being driven single ended from a 10k to 100K pot typicall used in a preamp circuit. Thanks for your response.
Fred
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.