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KCC's Quizzes AQQ249 about a double-stage amplifier



		

			

									
					    KCC
					
							

			
				
									
											
			
			
				
  •  Analog Employees  

			
			
			    	
			                    
                
				                on Dec 14, 2023 
				
			
			
		


				
		

		    		    
		    
		    	
			
Dear Brainstormers,


This week, we are back with our fundamental domain which is electronic design. Therefore, dear engineers, please jump on this!


 


A 2-stage cascaded amplifier, shorted by a resistor R, is amplifying a signal from an ideal voltage source VSRC .


The opamps are considered as ideal.


Questions :


    

  1. What’s the input impedance seen from the source VSRC for R= 120kΩ and for R=30kΩ (when opamps are in linear mode)
    2. 

  2. Find R value to cancel the current (IS=0) from VSRC (we, here, suppose VSRC is such that opamps don’t get saturated)
    3. 

  3. What’s the values range for VSRC to still let the 2 opamps in linear mode?
    4. 

  4. What’s the values at the output of the 2 opamps when VSRC= +2 volts
    5. 



Good luck!


Kuo-Chang

										


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[edited by: emassa at 6:57 PM (GMT -5) on 15 Dec 2023]
				

						
			        
		


				
						
        

    
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                on Mar 13, 2025 5:29 PM
            
            
					
      
	
      

		
		
      
					
      1. Rin = 1 / (1/R1 - ((R2*R4) / (R1*R3) - 1) / R) where R1 = 10K, R2 = 40K, R3 = 10K, & R4 = 20K
      

      so when R = 120K --> Rin = 24K
      

      & when R = 30K --> Rin = -7.5K
      

      2. 70K 
      

      3. assuming perfect rail to rail opamps +- 1.5 V
      

      4. Op1 = -8 V, Op2 = 12 V (saturated)
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
				
						
				
				
		
		
      
			
		
      
	
      

      

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                on Mar 13, 2025 5:29 PM
            
            
					
        
	
        

		
		
        
					
        1. Rin = 1 / (1/R1 - ((R2*R4) / (R1*R3) - 1) / R) where R1 = 10K, R2 = 40K, R3 = 10K, & R4 = 20K
        

        so when R = 120K --> Rin = 24K
        

        & when R = 30K --> Rin = -7.5K
        

        2. 70K 
        

        3. assuming perfect rail to rail opamps +- 1.5 V
        

        4. Op1 = -8 V, Op2 = 12 V (saturated)
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
				
				
						
				
				
		
		
        
			
		
        
	
        

        

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