Typical Performance Characteristics

T_A = 25°C, V_CC+ = 16V, V_EE- = -16V, R_L = 2kΩ to 0V or mid-supply

Application Information

Use as a voltage output op amp

As a standard op amp, Bel amp has excellent performance characteristics across the audio spectrum and beyond. With 9MHz unity gain bandwidth and 124dB peak open loop gain,

High input impedance and low noise make it great for sensors such as thermocouples, IR sensors, biological, acceleration, pressure or other MEMs sensors. Combined with low distortion over the audio frequency band and stable, high current output, Bel amps are great for professional and consumer audio products.

Using Bel amp as a standard op amp typically means in a feedback loop, with 2kΩ or higher resistor load. C_C should be, depending on gain, between 10pF and 33pF.

Inverting op amp

Non-inverting op amp

Stability considerations for voltage output

In inverting mode, and in non-inverting mode with a gain of 2 or more, the circuit is stable with up to 0.01uF capacitance, so driving coaxial cables is not a problem. If non-inverting gain of 1 is needed, adding 100pF between pin 5 (outY) and ground gives this same stability for higher capacitance loads.

More than a “standard” op amp, Bel amp uses a unique complementary feedback pair (CFP) with the low noise and high input impedance of JFET input and the high gain typical of bipolar (BJT) input op amps. Coupling this CFP stage to a folded cascode current mirror of VAS keeps all nodes at a DC bias voltage, keeping distortion low. The cascode voltage is fixed with a low impedance A low impedance (0.5Ω) reference fixes the cascode bias point, has excellent thermal stability, and its low voltage drop increases input stage headroom compared to other bias methods.

Use as a current output amplifier

Bel amp’s stage 1 has, as noted in the circuit description, internal feedback and special circuitry to keep distortion low when used “open loop” with current output from outY. Thus pin 5 can be terminated into a resistor and used as a high gain, fully differential high impedance input amplifier for low level signals from sensors and transducers such as moving coil or moving magnet phono cartridges, guitar pickups, microphones, biomedical pickups, etc.

Differential input high gain low distortion transducer amp

This circuit has about 50dB of voltage gain with R_L=7.5kΩ and distortion near 0.01% if output current is kept below about ±300µA. Distortion increases as voltage at outY approaches maximum possible node voltages (≈2V from rails).

A high gain, low distortion amplifier for low level signals

For even better performance, a pair of Bel amps can be used in tandem. By terminating outY into the summing junction of a trans-impedance amplifier, the output current from the trans-conductance stage is fixed at virtual ground due to the feedback of stage 2. Thus node outY is at constant voltage, which reduces distortion further.

Two Bel amp 50dB (RL=7k5) gain stage with all internal voltage nodes held constant up to the output stage of the second op amp.

Distortion Reduction Tweak

Second harmonic distortion is caused primarily by mismatched currents in two halves of the first stage of an op amp. Bel amp provides a trim-pot to allow 2^nd harmonic distortion to be reduced down to the noise level by using a distortion analyzer or sound card with FFT. The trim-pot is visible in the photos of page .

Offset voltage reduction tweak

In voltage output mode, this same trim-pot allows offset voltage to be nulled. Clockwise increases the offset voltage. With both inputs grounded, offset voltage will measure near zero, use a “tweaker” screwdriver to adjust it to your requirements. It is factory adjusted to be within ±½mV prior to shipping.

Highest DC accuracy and lowest distortion can be set by injecting a low distortion differential sine wave into the trans-conductance amp and measuring distortion at Vout. Adjust the trim-pot on the first op amp for least 2^nd harmonic distortion. Then ground both differential inputs and, with a five or six digit voltmeter, adjust the trans-impedance op amp’s trim-pot to set Vout as close to 0V as you desire. This assumes bipolar power supplies are used. If using unipolar supplies, set Vout to mid-supply.

Power supply rails and output voltage range

Bel amp operates from ±20V down to ±2.5V, or unipolar 40V to 5V. With 2V overhead required for Vout, a 5V power supply range restricts linear operation to about ±1.5V output swing. Within this range, distortion remains very low, at about 0.0001%.

1. What are the key features that make Bel amp suitable for high-fidelity audio applications?

Bel amp is exceptionally well-suited for high-fidelity audio applications due to several key features. It boasts very low noise (4.5nV/√Hz input voltage noise density and 0.5pA/√Hz input current noise), which is crucial for amplifying low-level audio signals without introducing hiss or hum. Its incredibly low distortion (0.0001% THD in inverting mode, 0.0002% in non-inverting mode at 3VRMS, 1kHz) ensures that the amplified signal remains true to the original, preserving sonic purity. The high input impedance JFET (Junction Field-Effect Transistor) input stage is beneficial for audio sources that prefer not to be loaded down, such as phono cartridges or high-impedance microphones. Additionally, its ability to output up to 100mA current, coupled with a low output impedance (<10mΩ at 1kHz), allows it to drive various loads, including headphones and power amplifier input stages, effectively and stably even with capacitive loads up to 0.01µF.

2. How can Bel amp be configured for different types of signal amplification?

Bel amp offers versatile configuration options for different signal amplification needs, thanks to its dual output capabilities. It can be used as a standard voltage output op amp via pin 6 (outZ), suitable for typical feedback loop applications like inverting and non-inverting amplifiers with high input impedance and low noise. This configuration is ideal for general audio applications, sensor interfaces, and precision measurement systems.

Alternatively, it can function as a current output amplifier (also known as a transconductance amplifier) using pin 5 (outY). In this mode, the input stage generates a current proportional to the differential input voltage. This current output can be terminated into a resistance (typically 2kΩ to 10kΩ) to convert it into a voltage, offering high gain and low distortion for low-level signals from transducers like phono cartridges, guitar pickups, or microphones.

For even better performance and reduced distortion, two Bel amps can be used in tandem: one as a transconductance amplifier (current output) feeding into a second Bel amp configured as a transimpedance amplifier (voltage output). This "dual configuration" holds the output current of the first stage at a constant virtual ground, minimizing distortion further.

3. What is Bel amp's power supply flexibility and what are the implications for output swing?

Bel amp offers wide power supply flexibility, supporting both unipolar and bipolar configurations. It can operate with a unipolar supply from +5V up to 40V, or a bipolar supply from ±2.5V up to ±20V.

The power supply voltage directly impacts the maximum linear output voltage swing. The datasheet specifies that the output voltage (Vout) requires 2V of headroom from the supply rails. This means that if operating with a 5V unipolar supply, the linear output swing is restricted to approximately ±1.5V (5V supply - 2V headroom = 3V total swing, or ±1.5V from mid-supply). Similarly for bipolar supplies, a ±2.5V supply would allow for a maximum output swing of approximately ±1.5V (5V - 2V headroom = 3.0V) around 0V (a.k.a. “ground”)..

Conversely, using the maximum bipolar supply of ±20V would allow for a robust output swing of approximately ±18V. Within the linear operating range defined by the supply and headroom, distortion remains very low (around 0.0001%).

4. Explain the "trim-pot" feature and its purpose in Bel amp.

Bel amp includes a "trim-pot" (trimming potentiometer) that serves a dual purpose: reducing second harmonic distortion and nulling input offset voltage.

In transconductance mode (current output from outY), second harmonic distortion can primarily be caused by mismatched currents in the two halves of the input stage. The trim-pot allows for balancing these input stage halves, enabling the user to reduce 2nd harmonic distortion down to the noise floor by adjusting it while monitoring with a distortion analyzer or FFT-equipped sound card.

In standard op amp mode (voltage output from outZ), the same trim-pot can be used to adjust the input offset voltage. Input offset voltage is an inherent DC error that can cause a small DC voltage at the output even when the inputs are grounded. The trim-pot provides a means to null this offset, typically by adjusting it to achieve 0V output when both inputs are grounded (or mid-supply for unipolar operation). The device is factory-adjusted for lowest offset voltage prior to shipment.

5. What are the key architectural elements contributing to Bel amp's high performance?

Bel amp's high performance stems from several advanced architectural elements:

• Compound JFET+PNP Complementary Feedback Pair (CFP) Input Stage: This unique design combines the advantages of JFETs (high input impedance, low noise) with PNPs (high gain typical of bipolar op amps). Emitter feedback within the PNP further reduces open-loop distortion.

• Folded Cascode Current Mirror for Voltage Amplification Stage (VAS): Coupling the CFP input stage to a folded cascode current mirror in the VAS ensures that all internal node voltages are fixed at a DC bias point. This "voltage variable junction capacitance" effect is significantly reduced, leading to high open-loop gain with exceptionally low distortion.

• Low Impedance Reference for Cascode Bias: A low-impedance (0.5Ω) reference is used to fix the cascode bias point. This reference offers excellent thermal stability and increases the input stage headroom by minimizing voltage drop compared to other biasing methods.

• Unity Gain "Diamond Buffer" and MOSFET Boosted Output Stage: The outZ voltage output is driven by a unity gain "diamond buffer" that provides the initial 5 to 10mA of output current. For higher current demands (up to 100mA), an output boost stage utilizing matched complementary high-current MOSFETs engages. This design ensures low output impedance and the ability to drive significant loads with minimal distortion, even into a capacitive load up to 0.01uF.

• Internal Feedback and Special Circuitry for Current Output Mode: The first stage incorporates internal feedback and dedicated circuitry to maintain low distortion when used in "open loop" current output mode (from outY), even with low-level input signals.

6. What safety and operational considerations should users be aware of when using Bel amp?

Users of Bel amp should be aware of critical safety and operational considerations:

• Absolute Maximum Ratings: Exceeding any of the absolute maximum ratings (e.g., supply voltage, input voltage, peak input current, operating temperature) can result in irreversible damage to the product and surrounding circuitry. Operation at these limits is not guaranteed.

• Output Short Circuit Protection: Bel amp includes an internal protection circuit that monitors output current and shuts off the output drive if the peak current reaches 100mA to prevent over-current damage. While continuous short circuit current is rated up to 120mA, users should still exercise caution to avoid prolonged short circuits.

• Load Stability: For voltage output mode, Bel amp is stable with up to 0.01µF capacitance in inverting mode or non-inverting mode with a gain of 2 or more. For non-inverting unity gain, adding 100pF between pin 5 (outY) and ground is recommended to ensure stability with higher capacitive loads.

• Distortion in Current Output Mode: While low distortion is a key feature, when using outY as a current output, distortion rises rapidly if the output current exceeds approximately ±400µA or as the voltage at outY approaches within 2V of the supply rails.

• Legal Disclaimer: Belleson explicitly states that their products are not authorized for use as critical components in life support devices or systems. Users assume all responsibility for consequences of misuse or misapplication. Specifications may also change without notice.

7. What is the typical gain and distortion performance when using Bel amp as a high-gain, low-distortion transducer amplifier with current output (outY)?

When using Bel amp as a high-gain, low-distortion transducer amplifier with current output from pin 5 (outY), it can achieve approximately 50dB gain when terminated into a 7.5kΩ load.

The distortion performance in this configuration is typically near 0.01%, provided that the output current is kept below approximately ±300µA. It's important to note that distortion increases rapidly as the output current exceeds this range or as the voltage at outY approaches the maximum possible node voltages (which are approximately 2V from the supply rails). For even better performance and further distortion reduction, a dual Bel amp configuration, where the current output is fed into a second Bel amp configured as a transimpedance amplifier, is recommended.

8. What types of loads can Bel amp drive effectively, and what are its output characteristics?

Bel amp is designed to drive a variety of loads effectively due to its robust output characteristics.

• Voltage Output (outZ): This output can deliver up to 100mA of current, thanks to a MOSFET boosted output stage that engages after the initial 5-10mA provided by a bipolar diamond buffer. It has a very low output impedance of <10mΩ at 1kHz (and 5Ω at 20kHz). This low output impedance, combined with stability for loads up to 0.01µF (or higher with specific compensation for unity gain non-inverting mode), makes it suitable for driving:

• Audio loads (e.g., headphone amps, power amp input stages)

• Coaxial cables

• Precision measurement systems

• Various sensor interfaces.

• Current Output (outY): This output delivers a maximum current of ±1.5mA, although distortion significantly rises above ±400µA. Its output impedance is very high, rated at 10,000 kΩ at 20Hz, dropping to 750 kΩ at 1kHz and 38 kΩ at 20kHz. This high impedance is ideal for driving resistive loads (typically 2kΩ to 10kΩ) to convert the current to a voltage in applications like:

• Moving coil/magnet phono cartridges

• Guitar pickups

• Microphones

• Biomedical pickups