Design of a broadband programmable high-gain amplifier for weak signals

    In automatic control and automatic measurement systems, some non-electrical parameters (such as temperature, speed, pressure) need to be converted into electrical signals through sensors. These weak electrical signals can be amplified to drive the measurement, recording mechanism or control actuator, thereby realizing automatic control or automatic measurement.
    The amplifier designed in this paper has the characteristics of low noise and high gain. The overall design requires the maximum voltage gain of the amplifier to be 80 dB, and the input voltage effective value Vi≤10 mV. When Av=60 dB, the peak-to-peak value of the output noise voltage VONPP≤0.3 V, and the 3 dB passband 0～5／10 MHz is optional. In the passband, the gain fluctuation is ≤1 dB, the load resistance is (50±2)Ω, the maximum output voltage sine wave effective value Vo≥10 V, and the output signal waveform has no obvious distortion.

1 System overall design and theoretical analysis
 

1.1 Overall system design
    This system can be divided into four modules, as shown in Figure 1. The first part is the input buffer and fixed gain amplifier module. The operational amplifier builds a voltage follower as an input buffer and increases the input impedance. The fixed gain amplifier part amplifies the input weak signal to a voltage range suitable for post-processing; the second part is the graded filter module, which is designed with two low-pass filters of 5 MHz and 10 MHz, which are switched by low-noise relays to meet the requirement of preset bandwidth; the third part is the controllable gain amplifier module, which realizes 80 dB dynamic gain change; the fourth part is the power amplifier module, which drives a 50 Ω load.

    The difficulties of this system lie in the two aspects of dynamic gain and power output, which are analyzed theoretically below.
 

1.2 Theoretical analysis of dynamic gain
    The design index requires a gain dynamic range of 80 dB, which is allocated to three modules: fixed gain amplifier, controllable gain amplifier and power amplifier. First determine the gain dynamic range of the controllable gain amplifier module, and then you can easily allocate the gains of the other two modules.
    Since the linear gain adjustment range of VCA810 is -40 to 40 dB, as shown in Figure 2. However, after actual circuit testing, when the gain is higher than 30 dB, the circuit is prone to self-oscillation. Here, a two-stage VCA810 cascade is used to achieve a dynamic range of -40 to 40 dB. The fixed gain amplifier gain is set to 10 dB. The gain of the post-stage power amplifier circuit with THS3001 as the core is set to 30 dB. Therefore, the gain adjustment range of the system is 0 to 80 dB.

1.3 Theoretical analysis of the expanded output of the op amp
    When the system load resistance is (50±2)Ω, the maximum output voltage Vo≥10 V, then from the formula P=U2/RL, it can be obtained that the maximum value of the system output power is Pmax=102/(50-2)=2.08(W). After amplification by the preamplifier and intermediate amplifier circuit, it does not have the ability to drive the load, and needs to be amplified by the final power amplifier circuit to meet the system's output power requirements.
    Considering that except for high-voltage op amps, the maximum output voltage of general op amps is only about ±12 V when the supply voltage is ±15 V, it cannot meet the requirement of outputting 10 V effective value. Here, the method of floating the op amp power supply voltage is used to expand the output voltage amplitude, and the complementary symmetrical power amplifier circuit is used to increase the output current.
 

The conventional power supply mode of the operational amplifier is shown in Figure 3(a). The power supply is directly taken from the regulated power supply as shown in Figure 3(b). The power supply voltage of the operational amplifier is no longer directly taken from the regulated power supply, but is provided by the transmitters of two transistors. R1=R2=R3=R4, ignoring the VBE voltage drop, the power supply voltage of the operational amplifier is:     Due to the effect of the transistor, the power supply voltage of the operational amplifier changes with the output voltage, but the difference between the positive and negative power supply voltages of the operational amplifier remains unchanged.
    

    Through Multisim software simulation, the maximum output voltages of the two operational amplifiers in Figure 3 are compared. Both circuits are set as inverting amplifiers with amplification of 10 times. The simulation results show that the maximum output of the amplifier with power supply connection in Figure 3 (b) is about ±24 V, and the maximum output voltage of the amplifier with conventional power supply connection in Figure 3 (a) is about ±12 V. Figure 4 (a) is the waveform of the maximum undistorted output voltage, when the input voltage is ±2.5 V and the frequency is 1 MHz. Figure 4 (b) is the output waveform when the same ±2.5 V and 1 MHz signal is input. It can be seen that the signal has been severely cut off and distorted.

2 Core Hardware Circuit Design
 

2.1 Variable Gain Circuit
    The variable gain module uses TI's high-gain adjustable broadband voltage-controlled amplifier VCA810, which has the following features: high gain adjustable range -40 to +40 dB, gain control accuracy ±1.5 dB (±0.9 dB for high-precision models), differential input single-ended output, constant gain bandwidth 35 MHz, dB/V gain linearity ±0.3 dB, gain control bandwidth 25 MHz, low output DC error less than 40 mV, high output current ±60 mA, and low operating current 24.8 mA.
    The working principle of VCA810 is to control the gain by applying a DC voltage to its pin 3. The functional relationship between the control voltage and the gain is:
    
    
    when Vc changes from 0 V to -2 V, the gain changes linearly from -40 dB to +40 dB. In addition, when the Vc voltage exceeds 0 V, the gain is -80 dB.
    The schematic diagram of the VCA810 circuit is shown in Figure 5. For the sake of system stability, two VCA810s are cascaded to achieve a dynamic range of 80 dB. There are two gain control modes: manual and preset.
    When working in manual mode, the Vc signal is provided through the potentiometer voltage divider. When working in preset mode, the gain value is input through the keyboard, and then the Vc signal is provided through the microcontroller D/A output. [page]

2.2 Power amplifier circuit

    This module is to amplify the power of the signal output by the previous controllable gain amplifier module, and finally drive the 50 Ω load resistor. As shown in Figure 6, the design here uses the op amp expansion output circuit analyzed in the previous article to first amplify the voltage of the signal, then amplify the current, and then complete the power amplification function.

    THS3001 has a conversion rate of up to 6500 V/μs, a -3 dB bandwidth of 420 MHz and good in-band flatness. At 110 MHz, the gain only drops by 0.1 dB; it has a settling time of 40 ns for large signal applications; the differential gain error is less than 0.01%, the differential phase error is less than 0.02%; the nonlinear distortion is less than -96 dB; and the power supply voltage can be selected between ±4.5 and ±15 V. The
    current amplification part uses a Class AB amplifier built with MOSFET IRF9610 and IRF610 to amplify the signal output by THS3001, and the circuit is shown in Figure 6. IRF610 and IRF9610 belong to the third generation of Power MOSFETs from Vishay, providing designers with a powerful combination of fast conversion, ruggedness, low on-resistance and high efficiency.

3 Testing
 

3.1 Test conditions
    At room temperature of 25℃ and under normal laboratory conditions, a sweep frequency meter was used to test the amplitude-frequency characteristics of the system, and a signal source and oscilloscope were used to test the amplification factor of the system.
 

3.2 Test plan and data
    A 0-15 MHz 1 V effective sine wave is generated by a function generator, and then a 10 mV effective signal is generated through a -40 dB attenuator. The amplifier preset gain is input by keystroke, and the input and output signals of the system are observed by a dual-trace oscilloscope. Figure 7 is a line graph of some test results. The maximum gain of the system is 80 dB, and the gain fluctuation within the band is less than 1 dB.

    When the gain is 40 dB, increase the input signal amplitude and observe the maximum undistorted output signal amplitude on the 50 Ω load. After testing, the maximum undistorted voltage peak-to-peak value is 42 V.
    When the gain is 60 dB, short the input terminal to the ground and measure the peak-to-peak value of the output noise voltage to be 0.2 V.
 

3.3 Analysis of test results
    From the above data, it can be seen that the system has good performance in terms of output power, and the output power can reach 4.41 W, exceeding the design index of 2 W. The maximum undistorted output voltage is 42 V peak-to-peak, which is 6 V less than the maximum undistorted output voltage peak-to-peak of 48 V simulated by Multisim software. Considering the actual situation of devices such as the incomplete symmetry of transistor parameters, this result proves that the improved op amp power supply circuit is effective. The variable gain range is 0-80 dB. If frequency compensation and front-to-back matching are added to the VCA810 module, the gain dynamic range can be further expanded. The distortion is still very small at 80 dB, and there is a possibility of further increasing the gain. The noise suppression characteristics of the system are good in the gain range of 0-60 dB. The errors in the test process mainly come from electromagnetic interference and the noise level of the signal source.

Conclusion
    The broadband amplifier uses the voltage-controlled gain amplifier VCA810 and the ultra-high-speed current feedback amplifier THS3001 as the core, and uses digital technology to achieve gain preset. The total gain range is 0-80dB, the passband is 0-15 MHz, and the maximum output voltage peak-to-peak value reaches 42 V. The preamplifier uses the low-noise voltage feedback broadband amplifier OPA2690 to greatly increase the input resistance. The post-stage power amplifier uses the op amp expansion and current expansion output circuit, which effectively improves the system's load capacity.