JFET-Opamp amplifier

[benidubber]

Hi!
does somebody know why the second circuit is noisier than the upper one?
I would like to make this circuit to work at different gains but when i put the condenser to connect the gain pot to ground i get much more white noise from the amp.
how do you build one of those with variable gain?
many thanks
sandro
Ps sorry for the italian words but was the same photo I've showed to a friend in italy.
SILENZIO is for silence and MOLTO PIU RUMORE means more noise

 

[abbey road d enfer]

The JFET bias changes probably a lot between one and the other configuration. Check the DC operating point of the FET in both cases.

 

[tv]

If you get lower noise with just a resistor, then use that.

If you can, configure it for the lowest gain required. Then add a rheostat (a pot) with a cap (like in your "noisy" schem) in parallel to that resistor. This way you could vary the gain...

 

[Samuel Groner]

Please upload reasonably sized pictures, thanks...

The JFET bias changes probably a lot between one and the other configuration. Check the DC operating point of the FET in both cases.

Indeed. Without the cap drain current should be (much) higher which results in lower voltage noise.

To make the second circuit work well you should DC-couple the opamp to the drain. This will require a positive DC bias at the noninverting opamp input. See e.g. lab_preamp_100x_lp_r1.pdf

If you get lower noise with just a resistor, then use that.

His approach seems much more worthwhile. If you understand *why* the difference is observed next time you safe much trial and error.

Samuel

 

[benidubber]

sorry for the picture size...
thanks Samuel for your reply,
and for all the teachings on your website and around here!
your art is brilliant.
It is a great luck to have you writing on this post..
so let me ask some question about the circuit you posted while I'm studying it.
how do you calculate the right DC voltage for the non inverting input and the relative values for current source and R4?
which is the function of C3?

sandro

 

[abbey road d enfer]

Voltage across R4 is 0.24uA x 20kohm = 4.8V
Voltage at + and - input of U1A are equal. So the voltage across R3 is also 4.8V. Current through R3 is thus stabilised at 1mA, so the collector current of Q2 and the drain current of Q1 are also stabilised at 1mA.
Voltage across R5 is 3.1V; Q2 vbe introduces 0.6V drop so Q1 vgd is stabilised at 2.5V.
In these conditions, the 2SK170 produces a set of operational characteristics that are close to ideal in terms of noise, transmittance, frequency response, input capacitance.
Current through R6 is the sum of Q1 emitter current (1mA) and R5 current (equal to current in Q3, 0.24mA) for a total of 1.24mA, so the output of U1A sits at 1.24 V below Q1 source, which is dependant on the FET's characteristics. According to specs, the Vgs of Q1 should be around 0.5V, so the output of U1A should sit at about -0.7V.

Capacitor C3 provides a low impedance path for U1A's non-inverting input. I suspect it is there for two reasons: preventing noise build-up due to the noise-current of the input and preventing pick-up of unwanted interference that could create instability (stray capacitance).

 

[benidubber]

wow! engineers, I'm very impressed!!!

It will take a month or an year to understand your exhaustive explanation of the matter,
but I HAVE to understand it!
maybe a simulator can help me in this, what do you think about?
I've always studied circuits empirically on breadboard, but I have no measuring equipment, so everything is quite random....
well, from today things will change!
I'll look for a simulator where I can see currents and voltages, and try to find out how to generate the right conditions for the JFET in the circuit I'm playing with.
for the moment I'll stick to the DC couple of drain and -, and the + bias (thanks!!)
and look for a solution without merely copying the bias arrangement.
have a nice day
bye
sandro

 

[abbey road d enfer]

A sim program won't help you understand better. You need to know by heart Ohm's law and Kirschoff's theorem and the Thevenin transform, when I say by heart, I mean you should be able to look at a simple schematic and without thinking have an idea of the order of magnitude of the operating points and basic operation. This comes with time and effort, but it's worth it. I've self-studied electronics from age 12 to age 15, before having more formal training at school, and my only test equipment was a multimeter. I ended up with a lot of misconceptions but I learned a lot through trial and error.
A sim program, such as LTspice (free on the Linear Technology website) is good to confirm that an actual circuit behaves as it should.

 

[PRR]

What abbey said. Get a $2.99 DMM. Learn and digest Ohm's Law.

Remember that ALL the common audio circuits were invented and perfected BEFORE simulators.

 

[benidubber]

thanks, I'll do what you said.
I'm studying by myself too and knowing at least what is important to study perfectly, is half of the job!
bye
sandro