OP295GPZ

The OP295GPZ from Analog Devices is a dual rail-to-rail output operational amplifier that combines the DC precision of bipolar input stages with the rail-to-rail output capability of CMOS, using a BiCMOS process. This unique combination makes it suitable for applications that require both high accuracy and full output voltage swing, particularly in single-supply and low-voltage systems.

The OP295 is the dual version of the family; the OP495 is the quad version with identical electrical characteristics. Both are specified over the extended industrial temperature range of -40 to +125 degrees C, making them suitable for automotive and harsh industrial environments.

The rail-to-rail output swing is the defining feature. The output can swing to within millivolts of both supply rails, enabling the full use of the available supply voltage range. For example, with a 5 V single supply, the output can swing from near 0 V to near 5 V with a 100 kOhm load, and to within about 0.4 V of the rails with a 2 kOhm load. This zero-in/zero-out capability is critical for single-supply applications where the signal range must extend to ground.

The bipolar front end provides lower noise (51 nV/sqrt(Hz) voltage noise density) and higher accuracy (300 uV max offset voltage) than purely CMOS rail-to-rail amplifiers. The input bias current of 8 nA typical is also significantly lower than bipolar-only designs. The offset voltage drift of 1 uV/degrees C typical ensures stability over the wide operating temperature range.

The 75 kHz gain bandwidth product and 0.03 V/us slew rate position the OP295 as a precision DC and low-frequency amplifier rather than a high-speed signal processing device. It is ideal for sensor conditioning, battery monitoring, servo control, and power supply feedback loops where DC accuracy and output swing are more important than bandwidth.

The ability to drive capacitive loads exceeding 300 pF without oscillation is a significant advantage over many CMOS rail-to-rail amplifiers, which can be unstable with even small capacitive loads. This makes the OP295 suitable for driving coaxial cables, MOSFET gates, and other capacitive loads that would cause instability in less robust designs.

The 15 mA output current capability enables the OP295 to directly drive power transistors and H-bridges, making it an efficient driver for actuators and motors. The output current is sufficient to drive the base of a power transistor or the input of a MOSFET driver without additional buffer stages.

The GPZ suffix denotes the 8-pin PDIP (N-8) package, suitable for through-hole prototyping and socket mounting. The OP295 is also available in SOIC-8 (OP295GSZ) for surface-mount production. The Z suffix indicates RoHS compliance and the extended industrial temperature range.

The OP295GPZ operates as two independent rail-to-rail output operational amplifiers using a BiCMOS (Bipolar + CMOS) process that combines the precision of bipolar input stages with the output swing of CMOS.

BiCMOS Input Stage: The input stage uses bipolar PNP transistors in a differential pair configuration. This provides low input offset voltage (300 uV max), low input bias current (8 nA typical), and low voltage noise (51 nV/sqrt(Hz)). The PNP input stage also allows the common-mode input voltage range to include the negative supply rail (ground in single-supply operation), providing the zero-in capability. The input stage is protected against differential overvoltage by back-to-back diodes with current-limiting resistors.

CMOS Output Stage: The output stage uses complementary PMOS and NMOS transistors in a common-source configuration. Unlike traditional bipolar output stages that have a VBE (0.7 V) drop from each rail, the CMOS output transistors can swing to within millivolts of the supply rails when lightly loaded. The output stage is the key to the rail-to-rail output swing: as the output approaches either rail, the corresponding MOS transistor enters the triode (linear) region and can pull the output very close to the rail voltage. With a 100 kOhm load, the output swings to within 50 mV of each rail at 5 V supply.

Gain and Compensation: The OP295 uses a single dominant-pole compensation scheme that provides unity-gain stability. The gain bandwidth product is 75 kHz, which means the open-loop gain drops to 0 dB (unity) at 75 kHz. The phase margin of 86 degrees ensures stable operation even with moderate capacitive loads. The high open-loop gain of 140 dB (1000 V/mV) provides excellent DC accuracy in closed-loop configurations.

Capacitive Load Stability: The OP295 is designed to remain stable with capacitive loads exceeding 300 pF. This is achieved through a carefully designed output impedance and compensation network that prevents the output pole from degrading the phase margin. In contrast, many CMOS rail-to-rail amplifiers become unstable with capacitive loads as small as 50-100 pF, requiring external isolation resistors or compensation networks.

Rail-to-Rail Output Operation: The output voltage swing depends on the load resistance. With a 100 kOhm load at 5 V supply, the output can swing from 0.05 V to 4.95 V (4.9 V peak-to-peak). With a 2 kOhm load, the swing reduces to approximately 0.4 V to 4.6 V (4.2 V peak-to-peak) due to the voltage drop across the output MOSFET on-resistance. At 3 V supply with a 2 kOhm load, the swing is approximately 0.5 V to 2.6 V (2.1 V peak-to-peak).

Single-Supply vs Dual-Supply: The OP295 operates equally well from a single supply (3 V to 36 V between V+ and V-) or a dual supply (plus or minus 1.5 V to plus or minus 18 V). In single-supply operation, V- is connected to ground, and the input and output ranges include ground. In dual-supply operation, the input and output ranges include the negative rail (V-), enabling zero-in/zero-out capability.