Level Detectors, Absolute Value, Peak and RMS

[mediatechnology]

Improved Absolute Value and Peak Detector Circuit Needs Only One Op Amp

Adding a single diode, D6, provides a peak detector output.
D5 compensates for D6's forward drop.
I raised R6 to 100KΩ to reduce gain errors at low level from D5's dynamic impedance.
R6 can be ∞.


Improved Absolute Value and Peak Detector Circuit Needs Only One Op Amp

Tip: Adding a 6K49 in series with the absolute value output along with a 10µF to ground provides a VU-like ballistic.

Next Up: I'll make D5 and D6 emitter followers to increase output and peak charging current.

[mediatechnology]

Current-boosted Absolute Value, Average and Peak Detector Circuit Needs Only One Op Amp

Adding two transistors lowers the output impedance and provides a high-charging-current peak output.
D5 in the previous drawings is replaced by Q1, an emitter follower.
Q2, which replaces D6, provides a high charging current peak detector that can capture a single-cycle 20 kHz burst. (C1=0.47µF)
R8 and C2 provide an average VU-type response.

Ideally D1-D3 are matched and thermally track.
Q1 and Q2 Vbe can also be matched.
The maximum peak output is limited by the maximum reverse Vbe of Q2. Input voltages of 5 to 6V peak (12V p-p) seems to be a reasonable limit.
I used some matched ZTX1053 which appeared to operate to about 8V.
The op amp is a TL072A.
An input coupling capacitor, not shown, is required for best low-level performance to block the prior stage's offset.


Current-boosted Absolute Value, Average and Peak Detector Circuit Needs Only One Op Amp


Current-boosted circuit. Absolute Value Waveform 3V/Div


Current-boosted circuit. Input versus Peak Output Waveform 3V/Div

Note that D1 and D2 are unmatched so there is some slight asymmetry.