AD797 discrete clone - comparison of seven variations

[tiefbassuebertr]

I have read the follow articles:
- Discrete AD797 up and running
Discrete AD797 up and running
- Help with improved Wilson current mirror (AD829)
Help with improved Wilson current mirror
- Folded cascode headphone amp (AD829, AD797 and LT1469)
Folded cascode headphone amp
- AD 797
Discrete AD797 up and running
original Datasheet (schematic page 11):
http://www.analog.com/static/importe...eets/AD797.pdf

then I looked by "circuitmaker" at some relationships between circuit variations and THD at low and high frequencies. At bottom the results - but please note, that is not really an AD797, because quiescent current is much more higher (quiescent current 90mA instead 8-10mA). There are no discrete bjt transistors on the marked with high ft and low current at the same time. Please note secondly follow: by this simulation all capacitors and resistors so as the voltage source (power supply) and input source (signal generator) are idealized (i. e. no internal resistance by the power supply). In additional there are no lead respective wire inductance and spread capacitance through PCB layout.
Only the effects of pure circuit - are to see.
In real life (few years ago) only the versions IV and VI (only BUF) I have tested (I have implement in commercial phono- pre- and power amps with bad sound) - so beginners should not be evaluate and not trying for diy.

Following parameters are valid for all versions:
Voltage gain = 10-times, input 1Vss, output 10Vss, wave form: sine wave.
All PDF images are directly comparable, after performing download of all pdf files.
Follow trend is to be noted that when decreasing circuit complexity the THD at high frequencies goes down and at low frequencies the THD values increase. Additional it is clearly to see by freqency-response without caps for compensation, that the nearly perfect circuit pcb layout is very hard to find (which cad software therefore there are on the marked- perhaps one of you knows more?).
Also of interest it could be, whether there are great differences by simulation results in other cad software (e.g. Orcad, Microsim or others). Perhaps one of you this one can check out.

[tiefbassuebertr]

A: AD-797, close by orig. circuit from datasheet page 11 - simulation results
1) AC Analysis (Frequency response) without Cc and Cn
2) AC Analysis (Frequency response) Cc: 220p/390 and Cn 220p/390
3) Transient Analysis 10 KHz (sine wave)
4) Fourier Analysis 10 KHz (THD)
5) Transient Analysis 200 KHz (destroyed sine wave)
6) Fourier Analysis 200 KHz (THD)

[tiefbassuebertr]

B AD-797 Var I (without Q12) - simulation results
1) AC Analysis (Frequency response) without Cc and Cn
2) AC Analysis (Frequency response) Cc: 70p/500 and Cn 20p/1K5
3) Transient Analysis 10 KHz (sine wave)
4) Fourier Analysis 10 KHz (THD)
5) Transient Analysis 200 KHz (sine wave)
6) Fourier Analysis 200 KHz (THD) K2: 1mV K3: 3mV

[tiefbassuebertr]

C: AD-797 Var II (without Q12 and I7) - simulation results
1) AC Analysis (Frequency response) without Cc and Cn
2) AC Analysis (Frequency response) Cc: 70p/500 and Cn 20p/1K5
3) Transient Analysis 10 KHz (sine wave)
4) Fourier Analysis 10 KHz (THD)
5) Transient Analysis 200 KHz (sine wave)
6) Fourier Analysis 200 KHz (THD) K2: 1mV K3: 3mV

[tiefbassuebertr]

D: AD-797 Var III (2xCurrent source instead Current mirror Q5/Q6, without Q12 and I7) - simulation results
1) AC Analysis (Frequency response) without Cc and Cn
2) AC Analysis (Frequency response) Cc: 47pF/1K, without Cn
3) Transient Analysis 10 KHz (sine wave)
4) Fourier Analysis 10 KHz (THD)
5) Transient Analysis 200 KHz (sine wave)
6) Fourier Analysis 200 KHz (THD) K2: 3mV K3: 4mV

[tiefbassuebertr]

E: AD-797 Var IV (1xCurrent source instead Current mirror Q5/Q6 Q12 and I7) - simulation results
1) AC Analysis (Frequency response) without Cc and Cn
2) AC Analysis (Frequency response) Cc: 33pF/1K, without Cn
3) Transient Analysis 10 KHz (sine wave)
4) Fourier Analysis 10 KHz (THD)
5) Transient Analysis 200 KHz (sine wave)
6) Fourier Analysis 200 KHz (THD) K2: 0,75mV K3: 1,25mV

[tiefbassuebertr]

F: AD-797 Var V (resistor instead Current mirror Q5/Q6 Q12 and I7) - simulation results
1) AC Analysis (Frequency response) without Cc and Cn
2) AC Analysis (Frequency response) Cc: 0,8pF new place, without Cn
3) Transient Analysis 10 KHz (sine wave)
4) Fourier Analysis 10 KHz (THD) K3: 20mV
5) Transient Analysis 200 KHz (sine wave)
6) Fourier Analysis 200 KHz (THD) K3: 20mV

[tiefbassuebertr]

G: AD-797 Var VI only buffer stage - simulation results
1) AC Analysis (Frequency response) without Cc and Cn
2) AC Analysis (Frequency response) Cc: 100pF new place, not use for 3) until 6)
3) Transient Analysis 200 KHz (sine wave)
4) Fourier Analysis 200 KHz (THD) no distortion by 10KHz
5) Transient Analysis 10 MHz (sine wave)
6) Fourier Analysis 10 MHz (THD) K2: 3,6mV K3: 2,5mV

[andy_c]

Hi,

There's more detail of the AD797 implementation available too. For example, see this post. That schematic was based on the one from Scott's AES article on the AD797, which has lots more detail than the datasheet has. For more information on that AES article, feel free to email me.

[janneman]

Hi,

I noticed that the spectrum displays have a linear vertical scale rather than the ususal log scale in dB. The linear scale makes it quite hard to compare curves and to appreciate the actual performance. Would it be possible to change those scales to log?

jd