Alternate Method Indicating Compression Indicating above-threshol
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THAT Corporation Design Note
Alternate Method Indicating Compression
Indicating above-threshold condition with discussed Design Note 113. method shown that design note simply turned when output threshold amplifier went below ground potential using half cheap, readily available comparator, LED, some resistors. circuit cheap simple, though recommended that comparator's trimmed, since maximum represented error While approach suited many users, others preferred indicator which displayed green under-threshold condition above threshold. This design note discusses three alternative approaches implementing this function using bi-color indicate either above below threshold. might expect, complexity these different approaches roughly proportional their functionality.
Symmetry 300k 4301P VGND VR22
20k0
VInput Threshold
Increase
20k0
Output
Control-Voltage
10k0 1N4148
Increase
383k
Gain
1k43 10k0
Increase
V1N4148
4k99
100n 4k99 590k
4301P
Compression Ratio
4301P
4301P 2M00
Compression Indicator
GREEN
Output
LM393
Figure hard knee compressor/limiter with bi-color indicate above/below threshold
Copyright 2002 THAT Corporation rights reserved.
Revision 11/10/03
THAT Corporation Design Note
Alternate Method Indicating Compression
Figure shows simplest approach using bi-color LED, uses just more resistor than circuit Design Note 113. this remaining implementations, we've used actual op-amp output derive signal indicate above threshold. When circuit below threshold, OA1's feedback loop closed through output sits about 0.6V above ground. When signal transitions above threshold condition, conduction stops feedback provided through Consequently, output swings rapidly level approximately -0.6 This large voltage transition `snap-action') completely swamps error U1A, result, there need consider trimming this error suggested DN113. Under below-threshold conditions, output high, current drive green provided through dumped directly into ground. When signal goes above threshold, output goes low, sinking current through drive LED, also absorbing current which still flowing through total current just about which guaranteed sink capability 393. diode currents different between green LEDs. green operates just over whereas operates just over effect this difference mitigated fact that LEDs more efficient that green LEDs, should noted that both LEDs being starved. past been difficult find high efficiency bi-color LEDs, this probably because difficult match intensity pairs production environment. Ground currents should special concern with this circuit, since this arrangement results ground currents that swing more than fast transitions Compression Indicator could result substantial current spikes, possibly resulting ground contamination. combat this problem, C1-3 included Output keep these currents local 393. Don't tempted delete C16, since this bypassing capacitor special considerations, should connected directly grounded side shown. Figure shows another method implementing this function, without dumping current into ground. This technique uses same number resistors previous method, uses both gates 393.
GREEN
LM393 LM393
Figure Compression indicator with ground current
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Corporation
THAT Corporation Design Note
Alternate Method Indicating Compression
positive input negative input connected output OA1, while other inputs grounded. This circuit relies aforementioned `snap-action' threshold amplifier overcome slight differential prevent both gates from sinking current same time. When circuit below threshold output high, high sinking current. With ±15V supplies, total current about with approximately this going through green LED. When circuit goes above threshold, action switches, there about current going through LED. Since current draw constant flowing directly between supplies, local decoupling simplified. electrolytic capacitors have been removed, that remains single ceramic capacitor between supplies. This single capacitor explicitly addresses switching transients 393, should connected directly between pins This arrangement assumes that there will other local decoupling ground, that's case, designer want that well. While solution Figure addresses issue ground currents, LEDs still driven rather current levels, though this acceptable many applications. Figure shows circuit which uses current more efficiently with addition resistor transistors. 2N3906s cross-coupled arrangement which steers current coming from into appropriate LED. Consider below -threshold case where high sinking current. pulls low, driving into saturation, which turn effectively shorts emitter shutting off. consequence, current through shunted through green LED, except base current situation reversed above threshold condition. This results about current driving LEDs, which optimal most devices.
Compression Indicator
VOA1 Output
2N3906
2N3906
GREEN
LM393 LM393
Figure Compression indicator with ground current optimal drive
Copyright 2002 THAT Corporation rights reserved.
Revision 11/10/03
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