Blackmer gain cell
The Blackmer gain cell is an audio frequency voltage-controlled amplifier (VCA) circuit with an exponential control law.
The four-transistor core of the original Blackmer cell contains two complementary bipolar current mirrors that perform log-antilog operations on input voltages in a push-pull, alternating fashion.
The circuit was used in remote-controlled mixing consoles, signal compressors, microphone amplifiers, and dbx noise reduction systems.
In the 21st century, the Blackmer cell, along with Douglas Frey's Operational Voltage Controlled Element (OVCE), remains one of two integrated VCA topologies that are still widely used in studio and stage equipment.
[2] The earliest solid-state VCA topology was an attenuator rather than an amplifier; it employed a junction field-effect transistor in voltage-controlled resistance mode.
[2]1989b The Blackmer cell was more precise and had a greater dynamic range that prior VCA topologies but it required well-matched complementary transistors of both polarity types that could not yet be implemented in a silicon integrated circuit (IC).
[4] The Gilbert and Dolby circuits were easily integrated in silicon[5][1] but the Blackmer cell had to be assembled from tediously selected, precision-matched, discrete transistors.
[2][4] To ensure isothermal operation, these metal-can transistors were firmly held together with a thermally conductive ceramic block and insulated from the environment with a steel can.
[6] This first generation of Blackmer VCAs had a very long service life; as of 2002, analogue consoles built around the original dbx202 "cans" were still being used in professional recording studios.
The ECG-101, which was designed by Paul Buff, contained only the core of a modified Blackmer cell – a set of eight matched transistors – and was intended for pure class A operation.
These circuits, like the original hybrid dbx ICs, were a small-volume niche product that was used exclusively in professional analogue audio.
[8] Typical applications include mixing consoles, compressors, noise gates, duckers, de-essers and state variable filters.
Additional positive or negative bias voltage VY applied between the bases of T1 and T2 converts the mirror into a current amplifier or attenuator[14].
[20] Its four-transistor core – the Blackmer cell proper – combines two complementary current mirrors that are wired back-to-back and operate in a push-pull fashion.
Operational amplifiers A1 and A2 perform same voltage-to-current and current-to-voltage converter functions as their counterparts in a unipolar log-antilog circuit,[20] and maintain virtual ground potential at the core's input and output nodes.
[4] The gain of the Blackmer cell has an inverse relationship with temperature; the hotter the IC, the lower the slope of exponential control law.
In dbx noise reduction systems and THAT Corp's analog engine, this is ensured by the physics of the Blackmer RMS detector, which is PTAT by design.
Each of the four legs of the modified core contains one NPN and one PNP type transistor; although they are still functionally asymmetrical, the degree of asymmetry is greatly reduced.
This improvement was invented by recording engineer Paul Conrad Buff and manufactured since 1980 as the monolithic ECG-101 IC by Allison Research and the identical TA-101 by Valley People.
The circuit, which was first produced as hybrid dbx202C in 1978 and as monolithic 2150/2151/2155 ICs in 1981, minimizes log-error distortion when the value of each feedback resistor equals the sum of equivalent emitter resistances on NPN and PNP transistors.
[29][30] The design of a Blackmer cell IC is a compromise favoring a specific combination of distortion, noise and dynamic range of gain settings.
[31] Temporal mismatches caused by thermal gradients are avoided by careful placement of core transistors and surrounding components on the IC.
[32] The residual mismatch of PNP and NPN mirrors is compensated for with trimming, usually by injecting a very small current into one of the core's two output transistors.
[30] The output amplifier A2 operates at fixed closed-loop gain, drives a benign constant-impedance load and does not degrade distortion.
[35] The input amplifier A1 drives a nonlinear feedback loop wrapped around the core and must remain stable at any possible combination of VX and VY.
[42] In class AB cores, off-state suppression of input signal, which marks the lowest end of control scale, reaches 110 dB at 1 kHz but deteriorates at higher audio frequencies due to parasitic capacitances.
These sources of feedthrough can be neutralized with capacitive coupling, leaving one undesirable DC component, input bias current of A1.
