Voltage Adder (DC Amplifier Type V2)

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It seemed prudent to re-investigate the Moog voltage adder circuits I first looked at in 2001, to see if there was a simpler way to provide simple voltage addition, while not using as many components as the Type X1 DC amplifier. In other words, a complete op amp was not necessary, a lot because the operating gain G would be -1 or less, and because feedback cures many ills for DC interstage coupling. The output requirements are much more modest (because it isn't driving heavy feedback, nor working as a line driver, or subject to highly capacitive loads).

Type V2 DC Amplifier Engineering Notes

I've been researching the 1960s Moog modular synthesizer designs for some time. Derived somewhat from the "standard" Moog voltage adder, this weekend, I designed and built a new Type V2 DC Amplifier. It worked amazingly well, and  it requires somewhat fewer components then the Type X1 DC amplifier.

One main difference between the Type X1 and Type V2 is that I wanted to explore again the 1960s Moog modular power supply voltages of +12V/-6V. The Type X1 was purpose-built for ±15V power supply voltages, but would likely work just as well with ±12V power (or similar), just with reduced output voltage compliance. The Moog designs are really interesting, in how they creatively used ± split supply voltages that have an absolute ratio of 2:1. Perhaps borne out of necessity in the mid-1960s, due to early silicon transistors which had a more limited BVceo, nonetheless this type of split supplies seems to offer creative potential today. 

This circuit initially oscillated, until adding the internal AC roll off components of 0.1 µF + 36 Ω between the collectors of Q1 and Q1. This is rather normal, because the gain has to be rolled off, otherwise there be zero phase margin at gain ≡ -1. In this case, after adding the AC compensation, with ±5V output swing, the Full Power Bandwidth of the circuit is ≈ 8 KHz. This bandwidth seems very adequate for a circuit that is intended as an accurate DC control voltage adder.

The design takes advantage of the 2N5088 (NPN) and 2N5087 (PNP) transistors, which have very high β at low collector currents. No transistor matching was done at all, and yet the ∆Vbe is quite well matched. From initial characterization testing, the general specs for this circuit otherwise are:

  • Vo compliance under 2kΩ load: -5.76V ≤ Vo ≤ +8.20V.
  • Vos ≤ |2mV|.
  • Ios ≤ |300nA|.
  • Ib ≈ 64nA.
±5V output into 2kΩ

A closer look at Vos. 
In this case Ib * Rin (=100 kΩ) + Vos contribute to the final output.

With RF modified to 49.9kΩ, the input range includes 0 to +10V.
The output, under RL=2kΩ, swings from 0 to -5V.

Similarly, with RF=49.9 kΩ, the input range also includes 0 to -10V.
The output, under RL=2kΩ, swings from 0 to +5V.







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