Project - EE 420L 

By Marco Muņiz,

05/08/2019    

Email : munizm1@unlv.nevada.edu

  

  

Lab description

  

Design a voltage amplifier with a gain of 10 using either the ZVN3306A or ZVP3306A (or both) MOSFETs and as many resistors and capacitors as you need.

 
Specifications:
   
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Project:
 
For this project, I will be using the Push-Pull circuit as the base for our Voltage Amplifier. I chose this amp topology because it works very well in sourcing, and syncing, current due to the fact that at any given time, at least one of the transistors will be on. Also, the push-pull topology is great for driving loads due to its small output resistance.
 
                                                                  file:///C:/Users/mmuni/Pictures/Lab%20Project/circuit.JPG
                                                                                               (Figure 1: Push-Pull Amplifier)
 
 
The first project spec we must consider in this design is the current draw with no input connected. We must draw no more than 1mA of current from a 9V power supply. In order to make sure that the current drawn is as small as possible, we will add resistors to the sources of both transistors(Nmos and Pmos). Specifically, they will be placed between the source and VDD for the Pmos and between the source and ground for the Nmos. This will help in limiting the current draw so we dont pull large amounts from the power supply.
 
The calculations for the needed resistance and capacitance can be seen in figure2 below.
 
          file:///C:/Users/mmuni/Pictures/Lab%20Project/calc1.JPG  file:///C:/Users/mmuni/Pictures/Lab%20Project/calc2.JPG
                                                       (Figure 2: Hand Calculations for Gain, Rin, Resistance, and Capacitance)
 
                                                                                      Let Current ID = 0.7mA
                                                                                      Let Rb = 1Gig Ohm
 

KPn = 0.1233      Vthn = 1.824V
KPp = 0.145       Vthp = 2.875V

  

     
The large capacitors on the sources were used in order to treat the sources as AC ground for simplification when doing the gain analysis for the circuit, this gain analysis is pictured above. On the left image above, we can see the relation between Vs and Vout which results in the gain equation. Since our Load Resistance and gain are fixed for this design, the only components that can be freely adjusted for the gain are (R1||R2). To find Rin, we find the relation between Vs, A (Gain), and the Current. We will set Rb to a fairly large number to, say 1Meg Ohm, and solve for Rin with the needed gain.
                                                                            
We already know the values of KP's and VTH for these devices from the spice model used in Lab 6 , we can set the drain current to a value under 1mA, say 0.7mA, and solve for the VGS, VSG, Resistors, and finally, the capacitors.
 
    
R16k Ohm
R2120 Ohm
Rin90k Ohm
Cin1.1 uF
Cout10 uF
Cs28 uF

VGS1.93V
VSG  2.97V
gmp14.2 mA/V
gmn13.1 mA/V
                                             (Figure 3: Tables with calculated component values)

Designed Voltage Amplifier:
     
For the final design, all calculated component values were implemented. Overall, the design did meet the calculation expectations, however, we did have to separate the Nmos source impedances. This was done to match the RC time constants on both sources so we wouldn't unintentionally restrict the signal from passing. In doing this, we now had the added benefit of being able to micro-adjust the gain value of the amplifier by making minor changes to R2 and R4 since those resistances dominated the parralels on the sources and controlled the gain.

                                               file:///C:/Users/mmuni/Pictures/Lab%20Project/final_circuit.JPG     
                                                                       (Figure 4: Final Voltage Amplifier design)          
   
Quiescent Current Draw:  
   
As we can see in the image below, the source resistors helped in limiting our current draw from the power supply, with no input signal.
                                     file:///C:/Users/mmuni/Pictures/Lab%20Project/source_current.JPG  
                                                  (Figure 5: Current draw from 9V Power Supply under quiescent conditions)     
   
file:///C:/Users/mmuni/Pictures/Lab%20Project/quiescent_current.JPG
 

Circuit Gain:
   
This plot shows a Small Signal AC analysis with Vout/Vs = A(Gain). At the design requirement of 100Hz, we see a gain of 20.9 dB = 11 (Gain), and reaching a maximum gain of 23dB = 14 (Gain) until 1Mhz.
          file:///C:/Users/mmuni/Pictures/Lab%20Project/Gain.JPG
                                                                   (Figure 6: Circuit Gain Plot) 
file:///C:/Users/mmuni/Pictures/Lab%20Project/exp_gain.JPG
Our Vin was set to 100mV, thus our experimental gain = 816m/100m =8.16. We tried increasing the gain experimentally by increasing the source resistances but the highest value we were able to attain was 8.16. The oscilloscopes were also pulling in some noise which was messing with our read input voltage, which would have affected our experimentally measured gain.
     
 We estimate our gain from the transient where Vout/Vin => 1.17V/100mV = 11.7 Gain
file:///C:/Users/mmuni/Pictures/Lab%20Project/transient.JPG
                                                            (Figure 8: Transient showing Vin and Vout) 

Input Resistance Simulation:
 
The input resistance Rin was found by using a Small Signal AC analysis to measure the input voltage Vin divided by the current through the input capacitor Cin. [Rin = V(Vin)/I(Cin)]. From the hand calculations, we found an input resistance of 90k Ohms. The simulation showed an Input resistance range from 83k ohm at 100Hz to 65k ohm up to 1MHz. The simulation shows a clear illustration of how the input resistance in the Push-Pull Amp is difficult to keep exact since the feedback continuously lowering our input resistance due to Rin = Rb/(1-A)
 
file:///C:/Users/mmuni/Pictures/Lab%20Project/Rin.JPG
                                                               (Figure 7: Rin Simulation Plot)
   

Max Output Swing:
   

The output swing was found by increasing the amplitude of the input voltage, which will increase the input current, and looking at the waveform when the output was clipped to its maximum and minimum values

 
For this design, the output swing simulation was roughly 4V. 

file:///C:/Users/mmuni/Pictures/Lab%20Project/output_swing.JPG 
                                                            (Figure 9: Output swing simulation plot)
 
 
  file:///C:/Users/mmuni/Pictures/Lab%20Project/Max_swing.JPG 
 
_____________________________________________________________________________________________________________
 
CalculatedSimulatedExperimental
Gain10118.19
Quiescent Current700 uA705 uA791 uA
Output Swing3.77V
1.34V

Power Draw = Iquiescent * VDD = 791uA * 9V = 7.12 mWatts 

    In conclusion, the amplifier design displayed met all of the requirements during simulations. However, the experimental results varied by a larger margin. Specifically, we did not reach our expected gain value of 11, nor did we see the same output swing, our quiescent current was fairly spot on.We may be able to increase this output swing by placing more devices in parallel with larger resistor values to help account for the larger current demand. This would require we supply more current which would put us closer to the 1mA current limit. Another improvement which can be made to this design is using a common source amp as the input. This would allow the designer to more accurately control the input resistance so that we do not have to deal with the push-pulls feedback decreasing the gain.
 

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