We lately noticed how sure audio energy amplifiers can be utilized as oscillators. This Design Concept exhibits how those self same components can be utilized for easy amplitude modulation, which is trickier than it may appear.
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The related machine is the TDA7052A, which we explored in some element whereas making it oscillate. It has a so-called logarithmic gain-control enter, the achieve in dBs being roughly proportional to the voltage on that pin over a restricted vary.
Nonetheless, we could desire a fairly linear response, which might imply undoing among the chip designers’ cautious work.
First query: why—what’s the applying?
The unique function of this circuit was to amplitude-modulate the ability output stage of an infrasonic microphone. That gadget generated each the sub-10‑Hz baseband sign and an audio tone whose pitch assorted linearly with it, permitting one to listen to at the very least a proxy for the infrasonics. The thought was to maintain the amount low throughout comparatively inactive durations and solely improve it in the course of the peaks, whether or not these had been optimistic or detrimental, in order that frequency and amplitude modulation would work hand in hand.
The 2 primary choices are to make use of the machine’s inherent “log” legislation (extra like antilog), in order that the perceived loudness was modulated, or to feed the management pin with a logarithmically-squashed sign—the inverse of the gain-control curve—to linearize the modulation. The previous is less complicated however sounded moderately aggressive; the latter, extra difficult however smoother, so we’ll focus on that. The gain-control curve from the datasheet, overlaid with real-life measurements, is proven in Determine 1. As a result of we’d like achieve to drive the speaker, we will solely use the higher, extra flexible, a part of the curve, with round 26 dB of achieve variation out there.
Determine 1 The TDA7052A’s management voltage versus its achieve, with the theoretical curve and sensible readings.
For correct linear efficiency, an LM13700 OTA configured as an amplitude modulator labored excellently, however wanted a separate energy output stage and at the very least ±6-V provides moderately than the only, cut up 5-V rail used for the remainder of the circuitry. An OTA’s accuracy and even precision should not wanted right here; we simply need the consequence to sound correct, and may minimize some corners. (The LM13700’s datasheet is filled with attention-grabbing functions.)
Subsequent query: how?
On the coronary heart of this DI is an attention-grabbing type of full-wave rectifier. We’ll take a look at it intimately, after which pull it to items.
If we take a paralleled pair of present sources, one inverting and the opposite not, we will derive a present proportional to absolutely the worth of the enter: see Determine 2.
Determine 2 A pair of present sources could make a novel full-wave rectifier.
The higher, inverting, part sources present in the direction of floor when the enter is optimistic (with respect to the half-rail level), and the decrease, non-inverting half does so for detrimental half-cycles. R1 units the transconductance for each levels. Thus, the output present is a perform of absolutely the worth of the enter voltage. It’s proven as driving R4 to provide a voltage with respect to 0 V, which sounds extra helpful than it truly is.
Typical full-wave rectifiers normally have a voltage output, saved on a capacitor, and representing the height ranges. This circuit can’t do this: connecting a capacitor throughout R4 merely averages the sign. To extract the peaks, one other stage can be wanted: pointless. By the best way, the unique ideas for this stage had been customary precision rectifiers with integrated or added present sources, however they proved to be extra difficult whereas performing no higher—aside from inputs under ~5 mV, the place they’d much less “crossover distortion.”
The utmost output voltage swing is restricted by the ratios of R4 to R2 (or R3). Extreme optimistic inputs will are inclined to saturate Q1, so VOUT can method Vs/2. (The transistor’s emitter is servoed to Vs/2.) With R4 = R2 = R3, detrimental swings saturate Q2, however the ratio of R3 and R4 implies that VOUT can solely method Vs/4. Q1 and Q2 reply in a different way to overloads, with Q2’s circuit folding again a lot sooner. If R2, R3, and R4 are all equal, the utmost unclipped voltage swing throughout R4 is simply lower than 1 / 4 of the provision rail voltage.
Rising R1 and making R4 a lot higher than R2 or R3 permits a higher swing for these detrimental inputs, however on the expense of elevated offset errors. Including an additional achieve stage would give those self same issues whereas needing extra components.
Making use of the present supply to the ability amp
Conclusion: This circuit is nice for sourcing a present to floor, however should you want a linear voltage output, it’s much less helpful. We don’t need linearity however one thing near a logarithmic response, or the inverse of the ability amp’s management voltage. Feeding the present by way of a community containing a diode can just do that, and the ensuing circuit is proven in Determine 3.
Determine 3 Schematic of an influence amplifier that’s amplitude-modulated utilizing the twin present supply.
The present supply is simply as described above. With R1 = 100k, the output peaks at 23 µA for ±2.5 V inputs. That present feeds the community R4/R5/D3, which suitably squashes the sign, prepared for buffering into A2’s Vcon enter. The achieve attribute is now far more linear, because the waveforms in Determine 4 point out. The TDA7052A’s Vcon pin usually both sinks or sources present, however emitter follower Q3 overrides that in addition to buffering the output from the community.
Determine 4 Some waveforms from Determine 3, displaying its operation.
To indicate the operation extra cleanly, the plots had been made utilizing a 10-Hz tri-wave to modulate a 700-Hz sine wave. (The goal utility would have an infrasonic sign—from, say, 300 mHz to 10 Hz—modulating a pitch-linear audio tone starting from about 250 to 1000 Hz relying on the sign’s absolute stage.)
Some additional notes on the circuitry
The values for R4/R5/D3 had been optimized by a strategy of heuristic iteration, which is fancy-speak for many fidgeting with trimmers till issues appeared proper on the ’scope. These labored for me with the units handy. Others gave comparable outcomes; absolutely the values are much less necessary than the general mixture.
R7 and R8 could seem puzzling: there’s nothing like them on the PA’s datasheet. I discovered that making use of slightly bias to the audio enter pin helps decrease the chip’s inner offsets, which in any other case trigger some (distorted) feedthrough from the management voltage to the outputs. With a modulating enter however no audio current, trim R7 for minimal sign on the output(s). The distinction is barely audible, but it surely exhibits up clearly on a ’scope as traces which are badly slewed.
The audio feed wants to return from a volume-control pot. Whereas it may appear extra apparent to include achieve management within the community driving A2.4—in spite of everything, that’s the first perform of that pin—that proved over-complicated, and launched but extra temperature results.
Temperature results! The present supply is (largely) freed from them, however D3, Q3, and A2 aren’t, and I’ve made no try to compensate for his or her contributions. The sensible resolution is to make R6 variable: a big, user-friendly knob labeled “Impact”, thus turning the issue into A Function.
A2’s Vcon pin sinks/sources some (temperature-dependent) present, so various R6 permits cheap, if handbook, temperature compensation. As a result of its setting impacts each the achieve and the a part of the achieve curve that we’re utilizing, the efficient baseline is shifted, permitting roughly of the audio similar to low-level modulating alerts to move by way of. Determine 5 exhibits its impact on the output at round 20°C.
Determine 5 Various R6 helps compensate for temperature issues and permits completely different audible results.
Don’t confuse this circuit with a “correct” amplitude modulator! However for taking an audio sign, modulating it fairly linearly, and driving the consequence instantly right into a speaker, it really works effectively. The precise consequence may be seen in Determine 6, which exhibits each the detected infrasonic sign ensuing from a gusty day and the audio output, whose frequency adjustments are invisible with the timebase used, however whose amplitude may be seen to trace the modulating sign fairly properly.
Determine 6 An actual-life infrasonic sign with the ensuing audio modulated in each frequency (too quick to indicate up right here) and amplitude.
—Nick Cornford constructed his first crystal set at 10, and since then has designed skilled audio gear, many datacomm merchandise, and technical safety equipment. He has ultimately retired. Principally. Form of.
Associated Content material
- Energy amplifiers that oscillate— Half 1: A easy begin.
- Energy amplifiers that oscillate—intentionally. Half 2: A artful conclusion.
- Revealing the infrasonic underworld cheaply, Half 1
- Revealing the infrasonic underworld cheaply, Half 2
- Extremely-low distortion oscillator, half 1: how to not do it.
- Extremely-low distortion oscillator, half 2: the true deal
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