Op amps in small-signal audio design - Part 4: Selecting the right op amp (JFET-input types reviewed)

The OPA2134 is a Burr-Brown product, the dual version of the OPA134. The manufacturer claims it has superior sound quality, due to its JFET input stage. Regrettably, but not surprisingly, no evidence is given to back up this assertion.

The input noise voltage is 8 nV/√Hz, almost twice that of the 5532. The slew rate is typically ±20 V/µs, which is ample. The OPA2134 does not appear to be optimized for DC precision, the typical offset voltage being ±1 mV, but this is usually good enough for audio work. I have used it many times as a DC servo in power amplifiers, the low bias currents allowing high resistor values and correspondingly small capacitors.

The OPA2134 does not show phase reversal anywhere in the common-mode range, which immediately marks it as superior to the TL072.

The two THD plots in Figures 4.36 and 4.37 show the device working at a gain of 3× in both shunt and series-feedback modes. It is obvious that a problem emerges in the series plot, where the THD is higher by about three times at 5 Vrms and 10 kHz. This distortion increases with level, which immediately suggests common-mode distortion in the input stage. Distortion increases with even moderate loading, see Figure 4.38.

Figure 4.36: The OPA2134 working in shunt-feedback mode. The THD is below the noise until frequency reaches 10 kHz; it appears to be lower at 5 Vrms simply because the noise floor is relatively lower.

Figure 4.38: The OPA2134 in shunt-feedback mode (to remove input CM distortion) and with varying loads on the output. As usual, more loading makes linearity worse. Output 5 Vrms, gain = 3.33×

This is a relatively modern and sophisticated op-amp. When you need JFET inputs (usually because significant input bias currents would be a problem) this definitely beats the TL072; it is, however, four to five times more expensive.

The OPA604 Op-Amp
The OPA604 from Burr-Brown is a single JFET-input op-amp, which claims to be specially designed to give low distortion. The simplified internal circuit diagram in the data sheet includes an enigmatic box intriguingly labeled 'Distortion Rejection Circuitry'. This apparently 'linearizes the open-loop response and increases voltage gain', but no details as to how are given; whatever is in there appears to have been patented so it ought to be possible to track it down.

However, despite this, the distortion is not very low even with no load (see Figure 4.39), and is markedly inferior to the 5532's. The OPA604 is not optimized for DC precision, the typical offset voltage being ±1mV. The OPA2604 is the dual version, which omits the offset null pins.

Figure 4.39: An OPA2604 driving various loads at 7.75 Vrms. Series feedback, gain = 3.23×

The data sheet includes a discussion that attempts to show that JFET inputs produce a more pleasant type of distortion than BJT inputs. This unaccountably omits the fact that the much higher transconductance of BJTs means that they can be linearized by emitter degeneration so that they produce far less distortion of whatever type than a JFET input [6]. Given that the OPA604 costs five times as much as a 5532, it is not very clear underwhat circumstances this op-amp would be a good choice.