Design of DC broadband amplifier based on MSP430

Abstract: Based on the voltage-controlled gain broadband amplifier VCA810, in combination with the low-noise high-speed op amp OPA820ID and the high-output voltage low-distortion op amp THS3091D, a broadband low-noise amplifier powered by a 5 V single power supply based on the ultra-low power microcontroller MSP430F155 is designed and manufactured. The VCA810 gain control and output waveform amplitude display are realized by the microcontroller software. The voltage gain is adjustable from -20 to +60 dB. At the maximum gain, the passband is 10 Hz to 10 MHz, and the peak-to-peak voltage of 29 V can be output under the load of 50 Ω. In order to solve the self-excitation problem of the broadband amplifier and reduce the output noise, various forms of anti-interference measures are adopted to suppress noise and improve the stability of the amplifier.
Keywords: broadband amplifier; VCA810; MSP430; single power supply; peak detection

With the development of social productivity, people are eager to be able to quickly and accurately transmit multimedia information anytime and anywhere over long distances. As a result, wireless communication technology has developed rapidly and the technology has become more and more mature. The broadband amplifier is an indispensable part of the above-mentioned communication system and other electronic systems. The low-noise amplifier circuit module determines the overall index of the system to a large extent. It can be seen that the broadband amplifier plays a very important role in the communication system, so people's requirements for it are getting higher and higher.

1 Overall design scheme
1.1 Technical indicators of broadband amplifier design
The design of broadband amplifier circuit mainly considers gain, passband, dynamic range, stability, etc. The main indicators designed here are as follows:
(1) Powered by a single 5 V power supply, the output is a 50 Ω resistive load;
(2) The amplifier voltage gain is greater than or equal to 40 dB (100 times), and the intra-band fluctuation is minimized;
(3) At the maximum gain, the lower limit cutoff frequency of the amplifier is not higher than 20 Hz, and the upper limit cutoff frequency is not lower than 10 MHz;
(4) At the output load, the maximum undistorted output voltage peak-to-peak value of the amplifier is greater than or equal to 10 V. When the amplifier input is a sine wave, the peak-to-peak value and effective value of the amplifier output voltage can be measured and digitally displayed.
1.2 Overall scheme description
The system composition block diagram is shown in Figure 1. The system is mainly composed of four parts: preamplifier circuit, controllable gain amplifier circuit, post-stage power amplifier circuit and single-chip display control module. The first stage amplifier circuit composed of OPA820ID has a gain of 6 dB, which realizes input impedance matching; the controllable gain amplifier circuit is composed of VCA810, which realizes the dynamic gain change of -40～+40 dB; the post-stage amplifier circuit has a gain of 14 dB; the single-chip display control module completes the control of VCA810 and the output voltage detection function and uses LCD to display the peak-to-peak value and effective value of the output voltage.

1.3 Circuit Design
The preamplifier circuit is composed of OPA820ID. OPA820 is a unity-gain stable low-noise voltage feedback operational amplifier with the following characteristics: high bandwidth (240 MHz, G=+2); high output current (±110 mA); low input noise; perfect current accuracy, 25℃ input bias voltage = ±750μV, input bias current is ±400 nA. The signal is input from the in-phase end, and the gain is (1+R17/R18) = 2 times, which is 6 dB. The circuit is shown in Figure 2.
The integrated voltage-controlled gain amplifier VCA810 produced by TI is selected as the main gain control. The gain of the voltage-controlled gain amplifier is linearly related to the control voltage, and the control voltage is generated by the DAC controlled by the single-chip microcomputer. VCA810 has a gain control range of -40 to +40 dB, an accuracy of 1 dB, and a bandwidth of 25 MHz. As shown in Figure 3.

The final amplifier circuit is composed of THS3091D, which is a current feedback operational amplifier with high output voltage and low distortion. It has the following characteristics: high bandwidth (210 MHz, G=2, RL=100 Ω); high output drive current (250 mA); low distortion and low noise; wide power supply range (5-15 V); the signal is input from the in-phase terminal, and the gain is 1+1 000/250=5 times. The output adopts two THS3091 in parallel to increase the driving capability. As shown in Figure 4.

The peak voltage detection circuit uses a high-frequency transistor to form an emitter follower detection circuit. The transistor envelope detector has a certain amplification effect, Kd>1, and also increases the input resistance Rid to (1+β) times that of the diode detector. The peak value detected is converted by A/D, and then the output voltage amplitude is displayed by the single-chip microcomputer. The detection circuit is shown in Figure 5.

The DC/DC constant voltage isolation non-regulated module A0512S-2W is used to convert the 5 V single power supply into ±12 V, and then the voltage regulator module is used to convert it into ±5 V power supply to power the first two amplifiers. The circuit is shown in Figure 6. Similarly, the constant voltage isolation module A0515S-2W is used to convert 5 V into ±15 V power supply to power the final amplifier. As shown in Figure 6.

1.4 Theoretical analysis and calculation
(1) Gain allocation. To achieve impedance matching, the first stage of the system is the input buffer stage. In order to expand the passband of the system, the input buffer stage gain is 6 dB. The gain adjustment range of VCA810 is -40 to +40 dB, and the highest linear gain error is only 0.3 dB/V. The gain of the final amplifier circuit is designed to be 14 dB, so the gain of the entire amplifier circuit is adjustable from -20 to +60 dB. The maximum peak-to-peak output voltage of VCA810 is 3.6 V. The gain of the post-amplifier is 5 times, which can make the maximum undistorted output voltage peak-to-peak greater than or equal to 10V.
(2) Passband analysis. The pre-amplifier chip uses OPA820ID. When its gain is 2, the bandwidth is 240 MHz and the bandwidth-gain product is 480 MHz. The bandwidth of VCA810 is fixed at 25 MHz, while the bandwidth-gain product of the final stage THS3091D is 420 MHz. When the gain is 2, the bandwidth is 210 MHz. From the above analysis, it can be seen that the upper cutoff frequency of the entire system is not less than 10 MHz. In addition, the three-stage circuit adopts DC coupling, and the lower cutoff frequency is not higher than 20 Hz. The high-speed, broadband operational amplifier selected in this system makes the gain of the signal in the passband flatter.
(3) Linear phase. In order to achieve linear phase in the entire passband, the impedance matching principle is strictly followed in the design, so that its load is purely resistive and a closed loop is constructed. Each integrated circuit is added with a decoupling capacitor to reduce the influence of parasitic inductance and capacitance.
(4) Suppress DC zero drift. Zero drift is the phenomenon that the input voltage is zero but the output voltage is not zero. Since the system is a broadband DC amplifier, DC coupling must be adopted between each stage. However, for high-gain amplifier circuits, the small input offset voltage of the previous stage will also produce a large bias after amplification. For broadband DC amplifiers, it is necessary to have good suppression performance for DC zero drift. The DC zero drift of the system is determined by the three stages together, and the bias of the previous stage circuit has a greater impact on the system. First, the system uses the unit-gain stable, low-noise broadband op amp OPA820ID to form the pre-amplifier circuit. Second, the system uses a hierarchical method to eliminate DC bias and connects VCA810 into a circuit with adjustable bias voltage.
(5) Amplifier stability. The system uses the following methods to reduce interference, avoid self-excitation, and improve the stability of the amplifier: wiring is done according to the signal direction, and coaxial cables are used for the connections between each level; the decoupling capacitor is as close to the chip power pin as possible; for the current-type feedback op amp THS3091D, special attention is paid to the wiring layout, such as the feedback line must take the shortest route, because a long line will cause a large additional phase shift; calculate and select the appropriate feedback resistor value so that it does not produce a large distributed capacitance due to a large resistance value, resulting in a large additional phase shift, nor does it reduce the bandwidth of the amplifier due to a large resistance value.

2 Software Design The MSP430 series of microcontrollers are a 16-bit ultra-low power mixed signal processor (Mixed Signal Processor) launched by the
American Texas Instruments (TI) in 1996. Because of its extremely low power consumption, rich on-chip peripherals and convenient and flexible development methods, it has become a dazzling new star in many single-chip microcomputer series. For the MSP430 series, the introduction of FLASH program memory and JTAG technology not only makes the development tools simple, but also relatively cheap, and can also realize online programming. The program is developed in C language. The flowchart of VG control and output voltage amplitude detection and display of VCA810 is shown in Figure 7 and Figure 8.

3 System test
3.1 Amplifier amplitude-frequency characteristics and gain fluctuation test
Test plan: The input signal is set to 100 mV, the control voltage VG is adjusted, and the amplitude-frequency characteristics of the amplifier are tested at different gains. The amplitude-frequency characteristics test results are shown in Table 1. The experimental results show that the bandwidth-gain product of the amplifier is basically equal to 3.36 GHz, the gain can reach more than 40 dB, the output voltage peak-to-peak value can reach 29 V, and within 10 MHz, the gain fluctuation is almost zero, the waveform is very smooth, and there is no self-excitation phenomenon. At 40dB gain, the lower cutoff frequency of the amplifier is lower than 20Hz, and the upper cutoff frequency is higher than 20MHz. The results are shown in Table 1.

3.2 Output noise test
The gain is adjusted to 40 dB, the input is short-circuited, and the effective value of the output noise voltage is measured to be 200 mV.

4 Conclusion
This paper presents the design of a broadband low-noise amplifier powered by a 5 V single power supply. The test results show that the gain, bandwidth, load capacity, and output voltage peak-to-peak value can meet the design requirements. In order to solve the self-excitation problem of the broadband amplifier and reduce the output noise, various forms of anti-interference measures are adopted to suppress noise and improve the stability of the amplifier.