SBOA305 October 2020 INA828 , OPA210 , OPA2210
Texas Instruments (TI) new generation of bipolar amplifiers are built on a precision complementary bipolar semiconductor technology that incorporates super-beta bipolar transistors. Super-beta transistors are optimized for high current gain (β > 1000) which helps reduce the device’s input bias current and input bias current drift over temperature.
This technology also incorporates advancements leading to better transistor matching and temperature stability yielding higher precision. This tech note will explain how super-beta transistors improve precision performance in bipolar amplifiers.
The beta (β) of a transistor, or transistor current gain, is the ratio of the transistor’s collector current (I_{c}) to its base current (I_{b}), as shown in Equation 1.
The β value is fixed for a given transistor and operating condition. Figure 1 shows a simplified amplifier input stage using bipolar transistors.
The value of β for traditional bipolar transistors typically ranges from 50 to 200 which may lead to a sizeable base current for a given collector current. For example, a transistor with a β value of 100 and I_{c} of 10 μA will require 100 nA of I_{b}.
In contrast, transistors with super-beta technology have a much higher β value, often well over 1000, greatly reducing the I_{b} needed for a given I_{c}. Revisiting the example above, for a transistor with a β value of 1000, the required I_{b} is decreased to only 10 nA. This is a 10x improvement over traditional bipolar transistors
In an amplifier, base current of the input transistors translates to input bias current (I_{B}). Traditional bipolar amplifiers offer better speed-to-power ratio and lower noise, however, they tend to suffer from higher I_{B} when compared to other silicon technologies like CMOS. The higher I_{B} may be unsuitable for high source impedance applications.
While CMOS and JFET amplifiers offer a clear advantage on I_{B}, new super-beta bipolar amplifiers offer significantly lower I_{B} while still providing the same benefits as the traditional process (speed-to-power ratio, lower voltage noise, lower 1/f noise, and lower open loop output impedance). To learn more about bipolar process advantages take look at the Trade-offs Between CMOS, JFET, and Bipolar Input Stage Technology application report.
Lower I_{B} translates to lower input current noise. The relationship between I_{B} and current noise is best described by Equation 2. The current noise (i_{n}) of a bipolar amplifier is given by
where q is the elementary charge (1.6 x 10-19 C). It follows that the lower I_{B} of a super-beta amplifier results in lower current noise.
In addition, lower I_{B} translates to higher input resistance (r_{in}). The small signal relationship between I_{B} and r_{in} is given by Equation 3.
Where g_{m} is the transconductance and V_{T} is the thermal voltage (equal to about 25 mV at 25⁰C). Equation 3 can be simplified to show the inverse relationship between r_{in} and I_{B}. It follows that the lower I_{B} of a super-beta amplifier will result in higher r_{in}.
Unlike DC offset which can be calibrated out, filtering noise is not a trivial task. In applications with high source impedance, i_{n} translates to high input voltage noise density (e_{n}). For example, 2 pA/√Hz of i_{n }typically found in traditional bipolar amplifiers may seem acceptable. However, with a 1 MΩ source impedance the i_{n} translates to 2 μV/√Hz of e_{n}, which may be inadequate for many applications. On the other hand, a super-beta amplifier like the OPA2205 and OPA2206 with an i_{n} of 200 fA/√Hz yields 0.2 μV/√Hz. An order of magnitude improvement.
A traditional bipolar amplifier with ultra-low noise of 1 nV/√Hz with the i_{n} from the example above in a 10 kHz filter bandwidth and a circuit gain of 10 yields 350 μV_{rms} of total noise (e_{n} + i_{n}). In contrast, a super-beta amplifier like OPA2205 with a higher noise floor (7.2 nV/√Hz) results in 41 μV_{rms } under the same conditions, an 8.5x improvement.
Applications such as medical instrumentation, life sciences, and vibration sensing often require the use of high resolution analog-to-digital converters (ADCs) at rather low frequencies. Super-beta amplifiers such as TI's OPA2210 with low 1/f noise (90 nV_{pp}) and OPA2202 with an ultra-low 1/f noise corner (0.1 Hz) help reduce total errors at the output of the amplifier interfacing the ADC.
Texas Instruments offers an extensive portfolio of precision bipolar devices. Table 1-1 compares the INA118, an instrumentation amplifier with traditional bipolar transistors, to the INA818, a precision instrumentation amplifier with super-beta transistors. Thanks to the super-beta inputs of INA818 the current noise is reduced by a factor of almost 20, and I_{B} is reduced by a factor of about 10.
Table 1-3 highlights some of TI’s precision amplifiers with super-beta technology. For a full list, see our parametric search tool results by visiting ti.com/amps
Super-beta devices |
Description |
---|---|
THP210 (3 V to 36 V) |
Industry's first high voltage fully-differential, low noise (3.7 nV/√Hz) amplifier and ADC driver. |
INA848 (8 V to 36 V) |
Ultra-low noise (1.3 n/√Hz), high speed (45V/μs, 2.8 MHz), high precision amplifier with fixed gain of 2000. |
INA818 (4.5 V to 36 V) |
Low power (350 μA), high precision (35 μV), low noise instrumentation amp with over-voltage protection (gain pins 1, 8). |
INA819 (4.5 V to 36 V) | Low power (350 μA), high precision (35 μV), low noise instrumentation amp with over-voltage protection (gain pins 2, 3). Now available in 3 mm x 3 mm QFN package. |
INA821 (4.5 V to 36 V) |
Wide bandwidth (4.7 MHz), low noise (7 nV/√Hz), high precision (35 μV) instrumentation amp with over-voltage protection. Now available in 3 mm x 3 mm QFN package. |
OPA1637 (3 V to 36 V) |
Fully differential, Burr-Brown™ Audio amp with low noise easily filters and drives fully differential audio signal chains. |
OPA2210 (4.5 V to 36 V) |
Ultra-low noise (2.2 nV/√Hz), high precision (35 μV), wide bandwidth amplifier (18 MHz). |
OPA2205 (4.5 V to 36 V) |
High precision (25 μV), low power (250 μA), low noise, e-Trim^{TM} amplifier. |
OPA2202 (4.5 V to 36 V) |
High capacitive drive (25 nF), ultra-low 1/f noise corner (0.1 Hz), precision, dual amplifier. |
OPA207 (4.5 V to 36 V) |
Precision, low noise amp replaces industry standard OP-07, OP-77, and OP-177 with higher speed (1 MHz) and lower power (375 μA). |