Using the AD844 as an I/V

[dirkwright]

Originally Posted by georgehifi

What do you guys think of this, it's a discrete clone of the AD844 with a different buffer, but it looks to me like the distortion rises below 2khz and what is the y axis measurments?
Cheers George

I don't understand why you need to two sets of current mirrors in the IV stage. I tried modeling one similar to this, from the pencil sketch image from a previous post. I don't get good results. THD is too high. It appears that the TZ resistors are loading down the IV stage because the THD went down when the resistor values went up. One thing that could be done is to beef up the IV stage so that it can drive lower resistor values without adding distortion. I don't think the distortion is coming from the diamond buffer, in other words. I also found that DC balance was not that good, but these kinds of amplifiers are known for not having accurate DC response, according to what I have read.

[MiiB]

The second mirror set is for the buffer in the AD844... but cascodede mirrors also maintain a higher impedance and thus better distortion specs. for even better you can introduce a helper transistor.

If you want better performance you also need to have CCS's instead of the 3.5 Kohm current set resistors , they modulate the current under signal and will be a primary source of distortion

[georgehifi]

Don't know if you guys can understand this, I certainly cannot, but it seems to test test the AD844 quite heavily. If you have some comments please post them.
http://www.emo.org.tr/ekler/ef07f927172f6d4_ek.pdf

Cheers George

[dirkwright]

Originally Posted by georgehifi

Don't know if you guys can understand this, I certainly cannot, but it seems to test test the AD844 quite heavily. If you have some comments please post them.
http://www.emo.org.tr/ekler/ef07f927172f6d4_ek.pdf

Cheers George

I don't understand all of it, but it's interesting.

Trying to combine the functions of current to voltage conversion as well as low pass filtering with a current feedback amplifier is not an easy thing to do in my mind.

[georgehifi]

I think this may have been the way Charlie Hansen used multiple AD844's to get his Ayer CDP's I/V stages and LP filtering and buffering all dc coupled as well.

Cheers George

[georgehifi]

The designer of the AD844 Barrie Gilbert, answer below to letting us see the full schematic of the AD844, he may send them if he finds them. Cheers George


Hi, George.

Regarding the full schematics of the AD844,
I wish I could help, but it has been so many
decades back that I honestly don't expect to
be able to find them.

Barrie

[dirkwright]

Originally Posted by georgehifi

The designer of the AD844 Barrie Gilbert, answer below to letting us see the full schematic of the AD844, he may send them if he finds them. Cheers George


Hi, George.

Regarding the full schematics of the AD844,
I wish I could help, but it has been so many
decades back that I honestly don't expect to
be able to find them.

Barrie

So, it sounds like you are interested in making a discrete version of the AD844, is that right?

[AudioLapDance]

Originally Posted by georgehifi

Don't know if you guys can understand this, I certainly cannot, but it seems to test test the AD844 quite heavily. If you have some comments please post them.
http://www.emo.org.tr/ekler/ef07f927172f6d4_ek.pdf

Cheers George

I can't seem to link to it now, but I glanced through it before. If I recall: it was an interesting way to create an advanced filter topology using multiple feedbacks, nesting and utilized pin 5 for the gain. It minimized the number of caps (but at the expense of many 844s) and had multiple taps for different freq ranges (3-way xover). It utilized the current feedback of the 844 and had better AC performance (less distortion?) than many regular voltage feedback opamps.

I believe it is just a "regular" opamp filter and not utilizing the unique aspects of the 844 (besides pin 5 for gain) that we are for IV.

[georgehifi]

Originally Posted by dirkwright

So, it sounds like you are interested in making a discrete version of the AD844, is that right?

No just think it would sim better if all the components are put in as well.

Cheers George

[georgehifi]

More education if you could guys, looking at the AD844's buffer that can have feedforward around this output buffer from pins 5 to 6 so it can drive high capacitive loads, is it possible to reconfigure the feedforward to become a low pass filter instead of what AD did? Cheers George

" DRIVING LARGE CAPACITIVE LOADS
Capacitive drive capability is 100 pF without an external net-work. With the addition of the network shown in Figure 34, the capacitive drive can be extended to over 10,000 pF, limited by internal power dissipation. With capacitive loads, the output speed becomes a function of the overdriven output current limit. Because this is roughly ±100 mA, under these conditions, the maximum slew rate into a 1000 pF load is ±100 V/μs. Figure 35 shows the transient response of an inverting amplifier (R1 = R2 = 1 kΩ) using the feedforward network shown in Figure 34, driving a load of 1000 pF."