Function of R2

[studiostevus]

Hi,
Newbie alert:

I am planning on designing a anode follower as an output stage to my tda1541 dac. I was planning to use this topology, but was wondering about the purpose of R2, and how to calculate appropriate values. Is it a simple grid leak?




Morgan Jones does really describe the anode follower (don't know why), so any help is appreciated.

Thanks

[Gilgy]

R2 is there to provide negative feedback - good explanation here

Designing Single-Stage Inverting Feedback Amplifier

[DF96]

R2 is the feedback resistor. To a first approximation, voltage gain = -R2/R1 (like the inverting op-amp). In that circuit it is also arranged to be part of the grid leak via RL, but you could do it differently if you wanted.

R1 sets the input impedance - generally as low as possible for most current-output DACs. You could even try omitting R1, then you have a transimpedance stage (current in, voltage out). Input impedance would then be roughly R2/mu - which could still be too high for the DAC chip. Many circuits seem to get away with violating the DAC data sheet requirements - whether this is because DACs are more tolerant than their makers suggest, or people who fiddle with DACs are more tolerant of distortion than they realise I am not qualified to judge.

[Yvesm]

R2 is the feedback resistor wich sets the gain as being roughly (R1 + R2) / R1 providing that the gain you need is lower (says 1/20) than the Mu of the triode.
Output and input impedances decrease as much as this ratio is high so, without R1 (understand 0 ohms) the input impedance fall down and it's probably what your TDA1541 expects for good linearity.

Yves.

[DF96]

Not (R1+R2)/R1, but roughly R2/R1 as I said - assuming sufficiently high open loop gain (equivalently, mu and appropriate load impedances).

[Yvesm]

In you, I trust

[Miniwatt]

Which tube are you using?

[no gas]

And all this time I thought R2 was the little droid in Star Wars!

[studiostevus]

I have some 6n16 submini's as well as some 6n2p's...

I will try to use 6n16 first, but havent done the calculations yet as to gain.

[studiostevus]

Allright, so I have done the math, but not sure of some things.
img31.gif

1. The value of R2 seems very high, and does not fit with your role of thump R1/R2=gain. I have calculated it using the formula on the aikenamps website (Rf = ( ((Ri+Ra-(Ri*A))*Acl)-Ra ) / (A-Acl).

I will use 2 parallel tda1541, so i/v converter of 28R is appropriate. That means I will be looking for A=12.4=Vout/Vin to get to 2Vpp on the output.


2. I was wondering about the 2mA 'DC' offset that the tda produces. In this setup, will that be present on the output? as there is no cap to block dc coming from the grid.

BTW the 6n16 datasheet is here

Thanks for the help!!