CBFET operational amplifier AD843 and its application in impedance matching circuit

    Abstract: This article introduces the main functional characteristics of the CBFET (complementary bipolar field effect transistor) operational amplifier AD843 and its application in impedance matching circuits. The uniqueness and superiority of this chip are illustrated through the comparison of several impedance matching circuits, and finally an application example of AD843 is given.

    Keywords: In many cases in circuit design, impedance matching transformation is required to adapt to the matching between various chips or components. Traditional impedance matching is composed of discrete components, so the circuit interference is large, debugging is troublesome, the development cycle is long, and maintenance is difficult. However, some op amp integrated chips are far from meeting the requirements of wider frequency band and precise matching due to their structural design limitations (such as frequency bandwidth, input bias circuit, etc.). The CBFEY (complementary two-stage field effect transistor) operational amplifier AD843 can meet more precise application situations with its unique CBEFT design. This article briefly introduces the characteristics of AD843, and combines the author's experience and comparison of several impedance matching circuits to demonstrate the superiority of the impedance matching circuit composed of AD843. Finally, a practical application circuit of AD843 in the author's virtual oscilloscope hardware design is given.

In addition to being used in impedance matching circuits, the AD843 can also be used in circuits such as high-speed sample-and-hold amplifiers, high-speed bandwidth active filters, high-speed integrators, and high-speed signal conditioners.

The main features of AD843 are as follows:

●The AD843 has the characteristics of high conversion rate, fast settling time and low input bias current, which makes it an ideal choice for 12-bit D/A and A/D buffer high-speed sample-and-hold amplifiers, high-speed integrator circuits and other applications. ideal amplifier;

●The unique internal structure of the complementary bipolar field effect transistor makes the AD843 feature high input impedance and low output impedance. Therefore, AD843 can replace many field effect transistor (EFT) input hybrid amplifiers, such as LH0032, LH4104 and OPA600;

●Adopts fully differential input mode, which makes it play a significant role in the regulation of standard high-frequency operational amplifier applications (such as high-speed bandwidth active filters, high-frequency signals, etc.);

●Adopts internal laser chip trimming technology, so that the input compensation voltage of AD843 can be lower than 1mV;

●You can use pin 1 and pin 8 of AD843 for external zero adjustment;

●AD843 does not require external compensation when running in closed loop.

1 Pin functions and characteristic parameters

1.1 Pin function

The CBFEF operational amplifier AD843 is packaged in an 8-pin DIP package. Figure 1 is its pinout diagram. The functions of each pin are as follows:

Pin 1 (BALANCE): external zero adjustment terminal;

Pin 2 (-INPUT): inverting input terminal;

Pin 3 (+INPUT): non-inverting input terminal;

Pin 4 (-Vs): negative power supply terminal;

5 feet (NO CONNECT): empty feet;

Pin 6 (OUTPUT): output terminal;

Pin 7 (+Vs): positive power supply terminal;

Pin 8 (BALANCE): Zero adjustment terminal.

1.2 Characteristic parameters

The AC and DC characteristic parameters of AD843 under ±5V power supply are as follows:

●Communication characteristics:

Unity gain bandwidth: 34MHz;

Fast settling time: 135ns;

Slew rate: 250V/μs;

Full power bandwidth: 3.9MHz;

Rise time: 10ns;

Input capacitance: 6pF.

●DC characteristics:

Input offset voltage: maximum 1mV;

Input bias current: typical value 0.6nA;

Input voltage noise: 19nV/(Hz)1/2;

Open-loop gain: 30V/mV when connected to a 500Ω load;

Output current: minimum 500mA;

Feed current: maximum 13mA;

Input resistance: 10 10Ω;

Output resistance: 12Ω.

2 Application design

2.1 Actual circuit requirements

Figure 2 is a block diagram of the front-end simulation part of a virtual oscilloscope hardware system designed by the author. The collection work is completed by the probe, whose input resistance is generally above 1MΩ, and the attenuation circuits of different proportions are composed of discrete original components. Gain adjustment is completed by the adjustable gain operational amplifier AD603. The input resistance of AD603 is about 100Ω, so the output resistance of the signal should be much less than 100Ω, otherwise the voltage division will be too large. In this way, the output resistance of the impedance conversion part is required to be small. Therefore, in order to prevent the signal collected by the probe from attenuating too much, the input resistance of the impedance conversion part should be about 1MΩ or greater. And the signal bandwidth is required to be above 10MHz. At the same time, it is required that the impedance transformation cannot attenuate the signal, that is, it has the function of a compressible follower. Moreover, the impedance transformation structure is also required to be simple and easy to debug.

2.2 Impedance matching circuit composed of discrete components

In order to find a suitable impedance matching circuit, the author debugged and compared several impedance matching circuits, including circuits composed of discrete devices and integrated chips.

The reason for using discrete devices is their low price. For this reason, the author designed a source output device, which is characterized by high input resistance and voltage amplification factor less than and close to 1.

Through experimental observation, it is found that in addition to the above advantages, the source output device composed of discrete components also has the following shortcomings:

(1) The waveform output by this source output device is very small.

(2) Static operation is difficult to set up, resulting in the negative half-cycle of the signal not being fully output.

(3) The frequency band is narrowed due to the influence of inter-electrode capacitance and wire capacitance.

In view of the above shortcomings, the author designed an offset circuit as shown in Figure 3, which uses twin FETs to reduce zero drift. Since a constant current source is used as the source load, the gain is closer to 1, and the DC working state is extremely stable.

However, it was found through experiments that this circuit also has the following shortcomings:

(1) It is not easy to find two N-channel junction field effect tubes with very symmetrical characteristics.

(2) Due to the influence of inter-electrode capacitance and wire capacitance, coupled with the inherent capacitance existing between each pole of the source load, the bandwidth is narrowed.

The impedance matching circuit of traditional analog oscilloscopes generally adopts the above two methods. It is also possible to set an emitter follower after the source follower or use differential amplification to solve the above shortcomings. However, too many components are required and the structure complex.

2.3 Impedance matching circuit composed of integrated chips

Integrated circuit chips have high integration, low interference, good performance, and small space occupation. Therefore, the author selected the low-power operational amplifier AD828 based on the requirements of high input impedance, low output impedance, and wide frequency bandwidth.

Figure 4 shows the pin arrangement of AD8228, and the impedance matching circuit composed of it is shown in Figure 5.

In Figure 5, D1 and D2 are voltage-limiting diodes, R1 is a 1MΩ resistor, and the operational amplifier AD828 is connected in the form of a voltage follower. It is observed through experiments that this circuit can meet the requirements in terms of frequency bandwidth. However, it still has the following shortcomings:

(1) When the input is open circuit, pin 3 of the input terminal of AD828 has a voltage of 1.0V, which causes pin 1 of the output terminal to not be zero. The adjustment requirement is: No matter the input is zero potential or the input is open circuit, the output terminal should be zero potential.

(2) If the resistance of R1 is reduced, the voltage at the input terminal of AD828 will also be reduced. This shows that there is a sink circuit in pin 3 of the input terminal of AD828.

(3) Due to the influence of sinking current, different voltage dividing resistors of different ratio attenuation circuits in the system will produce different voltages at the input pin 3 of the AD828, resulting in a voltage error at the output pin 1, which is difficult to eliminate.

Because none of the above circuits can meet the design requirements. Therefore, the author chose the AD843 chip and used the same circuit as the AD828 as shown in Figure 5 for impedance matching. After testing, it was found that when the input voltage is within the range of ±1V, its frequency band is around 20MHz, which can meet the frequency band requirements. And it has the characteristics of high input impedance and low output impedance, thus meeting the requirements of the system impedance matching circuit.

2.4 Specific circuit composed of AD843

We know that the waveform displayed by the oscilloscope can be moved up and down and zoomed in or out. Figure 6 is a specific circuit of the front-end simulation part of a virtual oscilloscope hardware system. In the figure, two chips, μP741 and AD603, are used to move the oscilloscope up and down and zoom in and out. The chip AD843 not only plays the role of impedance conversion here, but also undertakes the task of moving up and down the waveform displayed by the oscilloscope together with μP741. The coarse adjustment of waveform amplification and reduction is completed by attenuation circuits of different ratios, and the fine adjustment of waveform amplification and reduction is completed by AD603.

3 things to note

(1) Like most high-bandwidth amplifiers, the AD843 is susceptible to the influence of the load circuit, especially when used as a voltage follower. The load capacitance has little effect on the rated performance of the AD843 when it is less than 20pF. But when the load capacitance is large, the impact on AD843 cannot be ignored. Such as the peak situation of step response. At this time, a resistor or a combination of resistors and capacitors should be used to form feedback for adjustment.

(2) The static power consumption of AD843 is lower than that of many high-speed operational amplifiers, so a heat sink is not required. However, if the load resistance is too low, the circuit flowing through the load will increase, resulting in a significant temperature rise and an increase in the input bias current. A small radiator can be added at this time.

(3) The wire connections of the AD843 circuit should be as short as possible to provide low reactance and low inductance circuit channels to reduce coupling at high frequencies.

(4) Integrated circuit sockets should be avoided as much as possible because they may increase inter-line capacitance and reduce bandwidth.

(5) Two parallel capacitors of 2.2μF and 0.1μF should be added to the power supply terminal of AD843 to stabilize the power supply.

4 Conclusion

Through the comparison of several impedance transformation circuits and the practical application in the front-end simulation part of the virtual oscilloscope hardware system, the author believes that AD843 has obvious advantages in completing impedance transformation and can fully meet the design needs. It can greatly reduce the debugging workload and the trouble caused by interference. It is a new type of chip that is widely used.