Gain variable transimpedance amplifier integrated circuit for pulse laser rangefinder echo reception

Involves integrated circuit design technology, specifically refers to a gain variable transimpedance amplifier integrated circuit used for pulse laser range finder echo reception, which is used to convert the echo narrow pulse photocurrent signal of the pulse laser range finder and amplify.

Compared with other non-contact ranging methods, pulsed laser ranging has the advantages of simple structure, no need for target cooperation, long measurement range, and fast measurement speed, making it widely used in aerospace, military and industrial fields. The pulse laser rangefinder emits a light pulse to the target. After being reflected by the target, it is received by the rangefinder echo receiving channel. By measuring the time it takes for the light pulse to return to the laser rangefinder, the distance to the target can be obtained. distance. Pulse laser ranging combined with two-dimensional scanning or focal plane detection technology can also achieve laser three-dimensional imaging of the target. The echo receiving channel generally consists of a photodetector and a transimpedance amplifier circuit. The transimpedance amplifier circuit is used to convert and amplify the weak narrow pulse current signal output by the photodetector into a voltage pulse signal of a certain amplitude. Since the signal-to-noise ratio and rise time of the pulse signal output by the transimpedance amplifier circuit directly affect the single ranging accuracy of the pulse laser rangefinder, its gain, bandwidth and noise performance have a key impact on the performance of the entire rangefinder system.

Existing variable-gain transimpedance amplification circuits used for pulse laser rangefinder echo reception mostly use multiple chips to realize their functions. In this case, the requirements for high precision, high sensitivity and large dynamic range often lead to excessive power consumption and volume of the circuit, which is not conducive to the lightweight, miniaturization and mass production of the laser rangefinder system. At the same time, the external interconnection of the chip makes the circuit more susceptible to electromagnetic radiation interference.

The purpose is to provide a monolithic gain variable transimpedance amplifier based on CMOS integrated circuit technology that can be used for pulse laser range finder echo reception, and to solve the technical deficiencies existing in existing design methods.

The purpose is achieved through the following technical means:

Using CMOS integrated circuit technology, as shown in Figure 1, the circuit module includes a current buffer input stage, a differential transimpedance amplifier, an R-2R resistor attenuation network, a broadband voltage amplifier and an output buffer, where:

1) The current buffer input stage is the first stage of the circuit input. The basic circuit diagram is shown in Figure 2. The RGC circuit (Regulated Cascode Circuit) is used to achieve low input impedance and high output impedance, which effectively isolates the circuit input capacitance (including photodetector parasitic capacitance and interconnection line parasitic capacitance) and the input impedance of the differential transimpedance amplifier, reducing the The effect of circuit input capacitance on circuit bandwidth.
2) The differential transimpedance amplifier uses a differential amplifier plus feedback resistor configuration to convert the input current signal into voltage
Signal.
3) The circuit diagram of R-2R resistance attenuation network is shown in Figure 2. The resistors are implemented using NMOS tubes, which can overcome the bias level changes caused by the passive resistance attenuation network when direct coupling between stages. The bias level Vb is consistent with the output DC level of the previous stage. The 3-bit binary code digital control method is used to convert the 3-bit binary code into an 8-bit hot unique code, and the corresponding node output in the resistor attenuation network is selected to achieve 7 steps of 6-dB gain change per step.
4) The broadband voltage amplifier is composed of multiple transconductance-transimpedance voltage amplifiers cascaded. Transconductance-transimpedance amplifier, as shown in Figure 4, consists of two stages internally. The first stage is a transconductance stage, which provides transconductance for the input voltage signal. The second stage is a transresistance stage, and its feedback resistance is the transconductance of the first stage. Conductive current provides equivalent load. Using this circuit configuration, larger voltage gains can be obtained by increasing the feedback resistor. At the same time, since each pole of the circuit is not directly related to the feedback resistor, broadband applications can be achieved. Using multiple transconductance-transimpedance voltage amplifiers in cascade can provide sufficient post-stage voltage gain while ensuring a certain bandwidth.
5) The output buffer uses a source follower to achieve small output impedance and provide sufficient driving capability to drive off-chip resistive loads.

The working method is: the photodetector is coupled to the chip input pin through a capacitor, and the narrow pulse photocurrent signal generated by it is converted into a voltage signal by the current buffer input stage and the differential transimpedance amplifier, and then passes through the R-2R resistor attenuation network according to the control circuit After the given 3-bit binary control code is attenuated by the corresponding multiple, it is amplified to a certain amplitude by the broadband voltage amplifier and buffered and output through the output buffer.
Compared with existing technologies, it has the following advantages:
1) Using CMOS integration technology, a single chip implements a variable-gain transimpedance amplifier circuit, which can reduce the power consumption and volume of the circuit, and is conducive to the lightweight, miniaturization and mass production of the laser rangefinder system. At the same time, it is conducive to the realization of highly integrated multi-element laser echo detection.
2) Reduces electromagnetic radiation interference coupled through the chip’s external interconnects.
3) The gain of the transimpedance amplifier is digitally controlled, and its binary control code can be easily generated by the laser rangefinder digital control circuit, simplifying the gain control loop of the system.

Description of the drawings

Figure 1 is a schematic diagram of the basic structure of the present invention, which consists of five parts: a current buffer input stage, a differential transimpedance amplifier, an R-2R resistor attenuation network, a broadband voltage amplifier and an output buffer.

Figure 2 is a basic circuit diagram of the current buffer input stage of the present invention, using an RGC circuit (Regulated Cascode Circuit).

Figure 3 is a circuit diagram of the R-2R resistor attenuation network implementation.

Figure 4 Circuit diagram of a transconductance-transimpedance amplifier used in a broadband voltage amplifier. Among them, NMOS tubes M₁, M₂, I_S1 and load resistor R_D1 constitute the first stage of the circuit, that is, the transconductance stage. NMOS tubes M₃, M4, I_S2, load resistor R_D2 and feedback resistor Rf constitute the second stage of the circuit, that is, the transresistance stage.

Detailed ways

Example
Using 0.6-μm mixed-signal CMOS technology, design a gain variable transimpedance amplifier integrated circuit with the following input and output requirements: the input pulse photocurrent amplitude range is 0.1μA-10μA, the rise time is 4ns, and the circuit input capacitance is approximately 5pF. The required output pulse voltage amplitude range is approximately 1-2V.
In order to achieve higher ranging accuracy, the transimpedance amplifier circuit should keep the rising edge of the pulse signal as short as possible. According to signal theory, the minimum bandwidth required to maintain a 4ns rise time is approximately 110MHz. At the same time, according to the input and output signal amplitude, the maximum gain that the transimpedance amplifier needs to provide is at least 120dBΩ, and the gain control dynamic range is at least 40dB.

The specific implementation plans are as follows:

1) The current buffer input stage uses an RGC circuit. Adjust the parameters of each MOS tube so that the input impedance is about 100Ω. The pole formed with the 5pF input capacitor is outside 300MHz, which will not affect the overall bandwidth of the circuit. The output impedance is 20kΩ and is directly coupled to a differential transimpedance amplifier with low input impedance, ensuring very small level loss.
2) The differential transimpedance amplifier uses a differential amplifier plus feedback resistor configuration to convert the input current signal into a voltage signal. The feedback resistor uses an NMOS tube resistor, which can achieve higher transimpedance gain. With active output stage, output impedance 490Ω (single-ended), it can be directly coupled with the R-2R resistor attenuation network without much gain loss and bandwidth loss. A current-buffered input stage and differential transimpedance amplifier are cascaded to achieve 85dBΩ gain (differential) and 170MHz bandwidth.
3) The resistance in the R-2R resistor attenuation network is implemented by an NMOS tube. The on-chip bias voltage is added to the bias, and R is set to about 1kΩ. In order to reduce parasitic effects, the switching tube uses the smallest size NMOS tube. The input binary control word is 3 bits, which is converted into an 8-bit hot unique code by the 3-8 decoder, thereby achieving a total of 42dB step attenuation control. In order not to affect the overall bandwidth of the circuit, the R-2R resistor attenuation network buffers the output by a buffer to ensure the overall circuit bandwidth performance.
4) Transconductance-transimpedance amplifier in wideband voltage amplifier achieves 24dB (differential) voltage gain, approximately 230MHz bandwidth, using 2-stage cascade to achieve 48dB total gain and 160MHz bandwidth.
5) The output buffer uses a source follower circuit to achieve 100Ω output impedance and can directly drive off-chip loads. The entire transimpedance amplification integrated circuit achieves a maximum gain of 131dBΩ, 7 steps, a total gain dynamic range of 42dB, and a bandwidth of 120MHz.