PB50 high voltage power boost amplifier and its application

1 Overview
　　PB50 is a high-voltage field effect tube high-power boost amplifier launched by APEX Company in the United States. It has the characteristics of high operating voltage (200V), large output current (2A), high conversion rate (50V/μs), and low distortion. In addition, the current limiting circuit of PB50 can limit its internal power loss to a range that is almost the same as the safe area of ​​the power transistor. Good thermal conductivity allows it to obtain better power characteristics than ordinary power transistors.

　　PB50 is packaged in an 8-pin TO-3 iron box, which has good sealing and is internally insulated from each other. Its pin arrangement is shown in Figure 1. The functions of each pin are as follows:
　　Pin 1 (0UT): output;
　　Pin 2 (CL): current limiting feedback input;
　　Pin 3 (+Vs): positive power supply;
　　Pin 4 (IN): input;
　　Pin 5 (COM ): Common ground terminal;
　　Pin 6 (-Vs): Negative power supply;
　　Pin 7 (GAIN): Gain setting;
　　Pin 8 (C0MP): Phase compensation.

2 Pin characteristics and working principle
2.1 Basic connections of the boost amplifier
　　Figure 2 is the internal structure diagram of PB50. Because it is a high-voltage high-power amplifier with a relatively large output current, its setup and use are quite different from ordinary power amplifiers.

　　The power supply voltage requirements of PB50 are that the negative power supply must be 30V lower than the common ground terminal (pin 5), and the minimum operating voltage should not be lower than ±30V.

　　As can be seen from Figure 2, the input terminal IN of PB50 is a low impedance input. Typical value is 50kΩ. The maximum safe input voltage range must be limited to less than ±15V. The typical offset voltage of PB50 is 0.75V. Because it has a common-base bipolar input, when a pre-drive amplifier is used, this offset voltage will be subtracted from the output range of the pre-driver. For example: a pre-driver amplifier. The output voltage of the stage driver operational amplifier is 20V (peak-to-peak), and its actual output will be -10.75V and 9.25V. This offset voltage has no effect on the offset of the entire circuit. Because its offset voltage has been effectively offset by the open-loop gain of the pre-drive amplifier. But without a pre-drive amplifier, this offset voltage always exists. And it has a great impact on the imbalance of the entire circuit.

　　The common ground terminal "COM" provides a ground reference point for internal input and feedback. It should be noted that this port usually cannot be used as an inverting input terminal.

　　The gain pin "GAIN" allows an external gain resistor to adjust the closed-loop gain of the amplifier. The external gain resistor is connected in series with the on-chip feedback resistor. The calculation formula of the closed-loop gain is:
　　Av=[(RC+6200)/2100]+1

　　The frequency compensation pin "COMP" allows an external compensation capacitor, which is connected in parallel with the on-chip feedback resistor. Designers can estimate the frequency response of the PB50 by simply calculating the signal gain pole through the values ​​of the compensation capacitor, external resistor, and feedback resistor. The signal gain pole calculation formula is:
　　FP=1/[2π(RC+6200)/Cc]

　　where Rc is the external feedback resistor and Cc is the external compensation capacitor.

　　For example, a 22pF external compensation capacitor in parallel with a 6.2kΩ resistor will form a pole at 1.2MHz on the PB50's small signal response curve. A closed-loop gain of 10 can be achieved by connecting an external resistor of 22kΩ and a resistor of 6.2kΩ in series. If an external 22pF compensation capacitor is connected, the small signal response curve will begin to decrease at 260kHz.

　　The current limiting pin CL of PB50 is connected to an external current limiting resistor (RCL), which realizes current limiting through current negative feedback at the output end. The value range of the current limiting resistor is O. 27Ω~47Ω. On the premise of meeting the current output requirements, the current limiting resistor should be as large as possible. The relationship between the output current and the current limiting resistor value is:

　　+IL=(0.65/RCL)+0.1-IL=0.65/RCL

　　. In summary, the peripheral devices of PB50 should be connected according to the external connection diagram in its data sheet.

2.2 Design of composite boost amplifier
　　 When the system has very strict requirements on offset voltage, drift or bias current. A suitable preamplifier and PB50 can be used to form a high-performance, high-precision composite amplifier with a closed-loop structure.

　　When constructing a composite amplifier, the selection of the pre-driver must consider the following technical parameters: gain, stability, slew rate and output offset.

　　For maximum stability. The following principles must be followed:
　　(1) The boost amplifier should operate at the lowest possible gain;
　　(2) The pre-drive amplifier should operate at the highest possible gain;
　　(3) The gain-bandwidth product of the pre-driver should It is smaller than the gain-bandwidth product of the boost amplifier;
　　(4) The phase shift within the entire closed loop should be as small as possible.

3 Typical applications
3.1 Programmable power supply Programmable
　　power supply requires that the amplifier still has the characteristics of high linearity, low offset voltage and small bias current under high voltage and large current working conditions. In addition, in some specific cases, Quick responsiveness is required. The high-precision programmable power supply selects AD707 as the preamplifier. It has a very low offset voltage to obtain the best operating characteristics with a high-precision 18-bit DAC input. The output resistor divider scales the output down to the full-scale range of the DAC. Because it affects the accuracy of the circuit, high quality and low temperature coefficient (TC) resistors must be selected. If a hermetically sealed resistor network is used, the absolute temperature coefficient is not as important to the circuit as the temperature coefficient of the voltage divider resistor. When using an external DAC feedback resistor, it is best to use a voltage divider. The internal DAC feedback resistor can ensure the best temperature drift characteristics of the DAC itself. The temperature drift of most DACs using external feedback resistors can reach 300ppm/℃.

3.2 All the operating characteristics of the electromagnetic deflection amplifier
　　PB50 can only be realized at high voltage. Electromagnetic deflection applications require the amplifier to have a high slew rate at the current it can provide. Although the voltage in this operating state is not high, the PB50 can still meet the requirements. In the past, only some expensive integrated power amplifiers could meet both slew rate and gain bandwidth requirements.

　　PB50 can be used to form an electromagnetic deflection amplifier circuit. This circuit forces the coil current to be proportional to the input voltage by placing the deflection coil within a current feedback loop, in this case a feedback resistor to gain compensate the boost amplifier. Thereby improving the stability of the system. The auxiliary feedback network compensates for the 90° phase shift caused by the inductor and current sense resistor at high frequencies. to ensure stability during transient response. The fastest transient response time of any solenoid amplifier depends on its amplifier output voltage range and the coil's own inductance. When a voltage of nearly 140V is applied to a 200μH coil, the minimum transient response time is about 2μs, and the circuit composed of TL071 and PB50 can provide a conversion rate of 40V/μs. This means that it takes 4μs for the amplifier to provide full voltage output, resulting in a transient response time of less than 6μs for the circuit, equivalent to a scan speed of 83kHz.