The Research And Design Of Low Noise Pre-amplifier Based On Field Effect Transistor

The Pre-amplifier is referred to that the noise factor is very low, and serves as all kinds of radio receiver's Pre-amplifier generally, as well as high sensitivity electron detection equipment's pre-amplifier. When weak signals are magnified , the amplifier's noise interfere the signals seriously .Therefore, the amplifier's noise is hoped to be reduced in order to enhance the output signal-to-noise radio.The Pre-amplifier is generally in the front of the receiver, and it is mainly used to enlarge the weak signals received by antenna from the air, transient electromagnetism signals in the geological survey, as well as weak signals which are output from the sensor,and to reduce the noise disturbance,so the system can demodulate the message data needed.The design of the Pre-amplifier is absolutely important to the entire receiver,and its performance quality immediately influences the performance of the complete machine, particularly the receiver sensitivity.This article has studied the low noise Pre-amplifier in detail, particularly analysed the low noise pre-amplification circuit's characteristic and the design method,and designed the Pre-amplifier which is composed of two levels amplifying circuits. According to the design process ,this article has conducted the theory research, computation, simulation and welding and debugging of actual circuits, to the entire amplifier's first level of amplifying circuit, the second level of amplifying circuit and voltage-stabilizer circuit. Therefore, the whole text work is summarized as follows:1.Has studied the characteristics of the Field Effect Transistor(FET). To begin with the structure of FET, it has narrated the principle of FET, analyzed the control action of VG S to i D,and then introduced the characteristic curve as well as the related parameters. The FET's working condition may be divided into three regions: Variable resistor area, saturation area (constant flow area), puncture area. In this design, it is effective in the amplifying circuit and works in the saturation area. Finally it has used the small signal analytic method to analyze the amplifying circuit with FET,and established the small signal equivalent circuit diagram.2.Designs the first level of amplifying circuit using the difference amplifying circuit's principle. In difference circuit, both the change of temperature and the fluctuation of supply voltage can cause the same changes to currents of two collectors as well as the corresponding voltages, which is equal to join the syntype signals in two input ends. As a result of circuit's symmetry and the constant current bias, in the ideal situation, the output voltage might be invariable to restrain the drift of zero spot and the syntype noise. Therefore,it is well suited to use the difference amplifying circuit for the first level of circuit which requests to filter out the noise strictly. Using JFET 2SK152 as the core component to compose common source difference amplifying circuit,and then connecting two common-collector amplifying circuit in its output termination, first is used to enlarge the signal continuously to satisfy the design requirement that the first level of amplifying circuit enlarge 50 times, second is used to adjust the phase to make its output signal and the input signal have the same phase. Before the signal enters the amplifying circuit,it needs to filter the direct-current signal and add the protection circuit, to prevent the overburning of the chip caused by oversize signal.3.Use the principle of instrumentation amplifier to design of the second level amplifier circuit.Instrumentation amplifier is a closed-loop, differential input gain unit, used to accurately enlarge the input signal voltage.An ideal instrumentation amplifier enlarge the difference between the two input signal only,so there is a very high impedance between the two inputs and the ground.Amplifier output is single-ended-to-ground state,and equal to amplifier gain multiply the voltage difference accurately.Therefore, the first class amplifier output into the second-class amplifier can still maintain a low noise amplification.In instrumentation amplifier one input is connected to signals ,the other one is grounding.Use low-noise chip OP275G and AD847JN during the design process,and at the same time to ensure that the design bandwidth meets the bandwidth requirements.4.Disposing of the power supply of the circuit. Because it is low-noise amplifier, noise caused by DC power supplying must be filtered. Therefore, a voltage regulator circuit design should reduce the ripple factor of the power ,minimize the noise from the power supplying,and ensure the weak signal should be properly amplified,and not be submerged by noise. Firstly,use LM334 to compose the basic constant current source circuit, to ensure the current flows to the 2N5087 transistor constantly. Secondly, ,use 2N5087 to compose regulator circuit, and icrease voltage regulator tube in output. There can still be a stable output,even if the DC power supplying make noise±0.5V.Guarantee the stability of power.5.The design requires the work bandwidth of the circuit could be enlarged accurately in the range of 1M,and Minimize input noise.Therefore, in the whole process of design, the choice of devices should be considered in these two aspects, the bandwidth and low noise. The work bandwidth of first level amplifier core device (FET) meets the design requirements, and the using transistor is also a high frequency transistor.The noise of OP275 and AD847JN using in the second-class is also nV-class.The work bandwidth also meets the bandwidth requirements in the base of magnification.