Variable gain amplifier is a key module in GPS receiver. The automatic gain control circuit composed of variable gain amplifier and feedback loop provides constant signal power for analog-to-digital converter (ADC). The VGA gain of analog signal control gain changes continuously, but the linearity is poor.
Here, the negative feedback amplifier in the form of resistance is used to design an if variable gain amplifier with 0 ~ 30 dB gain change. The gain accuracy of VGA does not depend on the influence of process, voltage and temperature on resistance and MOS switch. The gain error is less than 5% at each process angle.
1 principle of variable gain amplifier
Analog circuits need to amplify or attenuate the signal, which can be realized by variable gain amplifier (VGA). It plays an important role in the analog front end of transceiver / transmitter in wireless communication. Fig. 1 is an analog front end diagram of a receiver for GPS. VGA at fundamental frequency compensates the gain attenuation of RF module and if module; VGA amplifies the output signal to the amplitude required by the A / D converter. AGC loop changes the gain of the receiver, adjusts the dynamic range of signals at all levels, and stabilizes the output signal power.
For VGA circuit, IIP3 and thd are important indicators because their output signal amplitude is very large. Secondly, in order to realize wide gain range adjustment and maintain constant output setup time under different gain input power, the gain of VGA is required to be DB linear with the control voltage. The smaller and more accurate the VGA gain step, the lower the requirements for ADC. In this paper, the digitally controlled VGA circuit provides a gain control range of 30 dB, uses 7b to accurately control the gain, and consumes less area and power.
Structure and performance comparison of variable gain amplifier
VGA is mainly divided into open-loop and closed-loop structures. A common open-loop structure is the Gilbert structure adopted in reference , as shown in Fig. 2. A reference voltage is added to MS, and the voltage VC controls the coupling current to change the gain. However, this structure circuit stacks four layers of circuits, which limits the swing of output voltage, and this circuit can not realize the control of exponential gain. Among the most widely used open-loop structures, variable gain amplifiers are mainly based on simple differential or Pseudo differential pairs, using source feedback technology, analog multipliers and MOS transistors connected by diodes as loads. The biggest problem of these structures is the linearity and distortion.
Because the negative feedback circuit can stabilize the output and reduce the nonlinear distortion, the closed-loop structure presents better linearity. VGA in common closed-loop circuit structure uses resistance array to realize gain control. For example, connect the resistance and MOS tube in series, control the on-off state of MOS tube switch, realize the change of resistance value, and then change the gain of amplifier. Because the resistance and MOS switch in the inheritance circuit are affected by process, voltage and temperature, it is difficult to achieve accurate resistance value, so the gain accuracy of PGA is limited. Reference  uses current division technology to realize accurate gain control. Reference  improves the resistance network, but these circuits are complex and additional circuits also increase power consumption. Here, a more accurate gain control is realized without increasing any design complexity.
3 high performance VGA structure and Implementation
In order to achieve the required gain control range and step size, two cascaded VGAs are used. The VGA of the first part realizes the gain control of 6 dB step, and the other part realizes the accurate O.5 DB step. Therefore, the whole VGA realizes coarse and fine tuning (see Figure 2).
When the gain of the operational amplifier is large enough, the gain of the closed-loop VGA is equal to the ratio of two resistors: gain = – RF / rs. changing the resistance can change the gain. The resistance value of coarse adjustment changes greatly. Changing the feedback RF will affect the poles of coarse adjustment output nodes; The resistance Rs is variable, which will form a variable load effect on the front stage. Select to change RS and add buffer circuit in the front stage for isolation.
First, consider the first stage 6 dB step gain: take RF = R0, rs = R1 to achieve a gain of 3 dB, then RF remains unchanged, rs = 2r1 to achieve a gain of 9 dB. Similarly: when rs = 4r1, 15 dB gain is achieved; When rs = 8r1, 21 dB gain is achieved; When rs = 16r1, 27 dB gain is achieved.
In order to better match, the switch size of MOS tube connected in series with resistance is designed according to the proportion in Figure 3. RS is equal to the on resistance and polysilicon resistance of MOS tube, and the on resistance of MOS tube is inversely proportional to w / L.
Considering the second stage O.5 DB step gain, it can be found that the conversion of O.95 to DB value is equal to -0.445 dB. 0.9 is -0.915 dB, 0.85 is -1.412 dB, o.8 is -1.938 dB, 0.75 is -2.499 dB and o.7 is -3.098 dB. An interval of 0.05 between 1 and 0.7 corresponds to an interval of approximately 0.5 dB in dB. Using this rule, the design can be as follows:
Two stage VGA can realize O ~ 29.5 dB (2.5 DB + 27 DB = 29.5 dB) gain control, and the step can reach O.5 DB accurately. Because the relative values of resistors are used in the design, the resistors and MOS switches are affected by factors such as process voltage and temperature, and the gain accuracy of VGA will be very small.
As shown in Fig. 4, the variable resistor R1 is realized by a polysilicon resistor and a MOS switch operating in the transistor region. Switching resistors are usually used in low distortion adjustable analog modules. The nonlinearity of MOS transistors will produce harmonics and intermodulation distortion, which will reduce the linearity of the whole circuit. In reference , an approximate formula is derived to approach the nonlinear characteristics of the switch.
The input voltage Vin is converted into a nonlinear current iin into a VGA amplifier in current mode. In weakly nonlinear networks, nonlinear harmonic distortion (HD2 and hd3) has been derived using Vol Terra series.
Where: Vin is the peak value of the input voltage; R1 equals R1 α+ Sum of RDS; α 2， α 3 is the quadratic and cubic nonlinear coefficient. Therefore, if the switch tube is placed at the virtual ground end of the operational amplifier (i.e. the input end of the operational amplifier), HD2 and hd3 are approximately equal to 0.
4 layout and post simulation results
Fig. 5 is an example of using SMIC 0.18 μ VGA layout realized by M CMOS process, chip area: 510 μ m × one hundred and sixty μ m. The whole layout includes VGA core, DC offset cancellation module, CMOS source follow buffer circuit and constant GM bias circuit.
Fig. 6 ~ Fig. 8 show the post simulation results of VGA simulated with spectre tool in the candence environment. Figure 6 shows the output transient response curve obtained by the input order jump.
Fig. 7 shows the VGA gain corresponding to different digital gain settings. Fig. 8 is the frequency domain response of the amplifier with different gains. Its gain changes from 0 dB to 29.5 dB, of which 0.5 dB is the first gear.
This paper introduces an o.18 μ M CMOS process, applied to the variable gain amplifier of GPS global positioning system. In this paper, the loop stability theory in feedback system is skillfully applied to design the amplifier; In the control of gain step, the gain is linearized with bit, and the gain accuracy is not affected by process angle deviation. The simulation results show that the amplifier is suitable for the analog front end of the receiver.
Responsible editor: GT