Discrete Op Amps

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Sparko’s Labs has created a family of discrete op amps in an 8 pin DIP compatible package optimized for high performance audio applications.   These devices are drop in replacements for many common, yet inferior audio op amps and are uniquely compensated for trouble-free swap out into virtually any circuit. Having a footprint of a mere 0.33 square inches, these devices are one-third the size of any other discrete op amp on the market.  Class A biasing and high output current capability coupled with a proprietary compensation scheme requiring multiple NPO dielectric capacitors make these discrete op amps impossible to fabricate as a monolithic IC.  These devices utilize matched transistor pairs encapsulated within a single device package for the input stage and internal current mirrors, which retains the advantage that monolithics have with device matching.  Each device is fully specified, and is available as a single or dual discrete op amp.

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                                                                                                   SS3601 Single Discrete Op Amp

Specifications SS3601 Single Discrete Op Amp Mechanical Data

  • Full Data Sheet Available Here
  • 140 dB Open Loop Gain
  • 415nV RMS (2.9nV√HZ) Noise to 20KHZ
  • +/- 18 Vcc Max
  • 15mA Class A Ouput Current
  • 65mA Max Output Current
  • 14mA Supply Current
  • < 300µV Typical Offset

Features

  • Optimized For High Performance Audio
  • Available In Single Or Dual
  • DIP8 Compatible Package
  • Circular Machined Gold Plated Pins
  • Smallest Discrete Op Amp On The Market
  • 100% Class A Biasing
  • Two Pole Compensation
  • NPO Capacitors
  • 1% Metal Film Resistors                                                       SS3602 Dual Discrete Op Amp

Applications SS3602 dual discrete op amp mechanical info

  • Preamps
  • DACs
  • Compressors
  • Sound Cards
  • Gain Stages
  • Mic Amplifiers
  • RIAA Amplifiers
  • Crossovers / Filters
  • Mixers
  • Line Drivers
  • Line Receivers
  • Any Op Amp In The Audio Path

 

Why discrete op amps? 

The SS3601 / SS3602 discrete op amps will outperform virtually all audio grade monolithic IC op amps in open loop gain, noise performance, output current, and magnitude of class A bias current. Even the coveted OPA627 monolithic op amp, with a price tag of over 25 dollars, has 30dB less gain and around twice the noise of these discrete op amps.   The table below details the superior performance of the Sparko’s Labs discrete op amps to an array of audio grade monolithic IC op amps.

Op amp comparison

Overview: All of Sparko’s Labs discrete op amps are based on Lin 3 Stage topology consisting of an input stage differential pair, a gain (VAS) stage, and an output stage all biased in class A mode with two pole compensation.  All active devices are Bipolar Junction Transistors (BJTs) for the greatest linearity and agility that any silicon device has to offer.     The devices are fully protected from over current conditions by active current limit circuitry in the output and gain stages, as well as being protected from large differential input voltages by back to back high-speed schottky diodes across the inputs.

Input Stage:  The input stage of these devices are comprised of a dual matched pair of NPN BJTs.  This means that the device’s inputs will pull a small input bias current (specified as Ib) that will flow into the device.   The common mode input voltage range of the input stage can be as high as a few volts below the supply rails, however the best performance is obtained by minimizing this to a few volts above and below ground in a split supply application. Input offset voltage is factory trimmed and typically turns out to be better than 250uV @  ±12Vcc.  The input stage is protected in the event that the inputs are driven apart, which usually happens during output clipping or rapid slewing.  A cascode Wilson current mirror  is utilized as the active load for the input differential pair for precise current matching between the input pair transistors.

 

SS3602_longGain (VAS) Stage:  The gain stage of the device is a cascode loaded Darlington for the highest linearity and open loop gain possible.  The cascode biasing voltage is derived from precision shunt references, which have a much lower dynamic impedance and lower noise than the low voltage zener diodes which are commonly used to derive this bias voltage.  The gain stage is current limited by diode clamping action as opposed to a feedback action, which results in greater stability during clip.

Output Stage:  The output stage is a push-pull emitter follower biased in class A mode with 8mA of standing current.  Due to push-pull action, the output stage can source or sink 16mA of current and still remain in class A mode.  The output stage will automatically revert to class AB mode in the event that more output current is demanded by the load, however the best THD performance will be obtained by ensuring that the output stage stays in class A mode.  Active current limiting is employed in the output stage to protect it from an over current condition.  The output transistors are high gain (β) individual devices in a SOT23 package manufactured by Diodes, Inc. who have developed a special manufacturing and encapsulation process that allows their devices to dissipate two to three times the power of a typical SOT23 packaged device. Utilizing these output devices allows Sparko’s Labs discrete op amps to have a high class A bias current and the ability to source or sink far more output current than comparable monolithic op amps in a DIP8 package.

SS3602_opamp

Compensation: All of Sparko’s Labs discrete op amps employ a uniquely implemented 2 pole compensation scheme that is extremely tolerant of capacitive loading and high feedback network resistance as seen by the input pins.  This allows these discrete op amps to be dropped into virtually any circuit arrangement and work without any stability issues.

Two pole compensation, despite its superiority to single pole schemes, is not often used in monolithic op amps due to the difficulty in fabricating the 2 capacitors at minimum that are required to implement it. Capacitors inside of monolithics consume a large amount of the die area and are therefore kept to a minimum in both capacitor value and quantity.  The amount of capacitance required for at least one of the two capacitors in a 2 pole scheme tends be impossibly large for monolithic designs anyway, even if the die area were available for two capacitors.
Beings how the Sparko’s Labs discrete op amps  employ 3
capacitors for compensation, they are impossible to
fabricate as a monolithic, and are only possible as a discrete op amp.

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Monolithic op amps mostly employ single pole compensation schemes.  They pay for this with a reduction in open loop gain at audio frequencies, as well as a reduction in maximum open loop gain that they can have in the first place.  Since compensation schemes burn off gain by nature, and since a single pole scheme burns it off at half of the rate of a two pole scheme, there is a limitation that exists in how much gain they can start out with in the first place to ensure they can burn it all off by the time the phase lag has shifted 180˚.  The monolithic op amps that DO have a high open loop gain always wind up having an excessively high bandwidth in the 50MHz region or so, which tends to make for a finicky device prone to instability and oscillation.  Such high bandwidth devices also suffer from more susceptibility to ill effects from layout parasitics, capacitive loading, resistive feed back networks, and usually require a more stringent power supply bypassing capacitor arrangement comprised of a tantalum and a
small value ceramic.  Such limitations give most monolithic op amps,
even the good ones, little chance of working as drop in replacements.

SS3601 Single Discrete Op Amp

Single Discrete Op Amp

SS3602 Dual Discrete Op Amp
Dual Discrete Op Amp

 

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