1 Application Brief

Introduction

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.

What is super-beta?

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.

Figure 1-1 Simplified differential amplifier input stage

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

Why do we care about base current?

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.

What are the benefits of lower I_B?

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 25?C). 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.

System level noise benefits

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.

How about a comparison?

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.

Where can I get a super-beta amplifier?

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