Products Mentioned

AD549 Production
The AD549 is a monolithic electrometer operational amplifier with very low input bias current. Input offset voltage and input offset voltage drift are...
Datasheet
AD549 on Analog.com

LTspice Production
LTspice® is a powerful, fast, and free SPICE simulator software, schematic capture and waveform viewer with enhancements and models for improving the simulation...
LTspice on Analog.com

ADA4077-1 Recommended for New Designs
The single ADA4077-1, dual ADA4077-2, and quad ADA4077-4 amplifiers feature extremely low offset voltage and drift, and low input bias current, noise,...
Datasheet
ADA4077-1 on Analog.com

Thread Details

  • State Not Answered
  • Replies 2 replies
  • Subscribers 36 subscribers
  • Views 55 views
  • Users 0 members are here
Options
 
Related Content

        

            Post
             Go back to editing
        

    

    


    

    









		


















	
	
			
	
			
		
	
	
		

	    
	    
	    		
Simulation with ADA4077 fails

		
	
				

			

									
					    Lippmann
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Jul 6, 2026 
				
			
			
		

		
		                     
				
        		

		    
		    		    

		      		            		        
Category: Hardware

    		        		            		    
Product Number:  ADA4077-1  

    		        		    		    

		    		    
		    			

			    
I am trying to do a noise analysis of a discrete JFET  input stage,  hybrid amplifier using two opamps in the feedback loop, one for setting the DC operating point of the JFET.  Using the AD549 model works nicely, but the ADA 4077 or similar, ADA 4620 fails.  I tried several different other OpAmps with no success.


Any ideas ?


 Noise_composite_FET_bias_control_1.asc 

			

			
									
		                             
                        

    

                                            
                 Operational Amplifiers (Op Amps)
                
                                                    
                 simulation
                
                                                    
                 amplifiers
                
                                                    
                 hardware
                
                                                    
                 LTspice
                
                                        
                                            
                
                
                                
                    Show More
            

    
    
		


				
				        
        		

			
						
									
												
			
			
                        
                                    
                        
                        
                        
            
			            
						
						
			
			
			
			
		


        
        
        
    
					

			





















	

	

					



			








	
	
	
	



Parents
	
    
	

  • 
	
    



	
		
	
	



    
	
    
		
    
							
					
    
		
    
		    
		    		    
			
				
									
							        
			
			
			
    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Jul 8, 2026 4:46 AM
            
            
						
			            		
    
		
										0
					
	
    

		
		
    
		
    
                                
                                                            
    
					
    Hello  ,
    

    I checked your schem and it seems like the 2nF cap on the feedback of the bottom amp.
    Can I know why you need that large cap at the feedback?
    

     
    

    Best Regards,
    Kenneth
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

    • 

	

  • 
	
    



	
		
	
	



    
	
    
		
    
							
					
    
		
    
		    
		    		    
			
				
									
							        
			
			
			
            
						
                            
            
            				
            
                on Jul 8, 2026 6:02 AM
            
            
												
						in reply to Kenneth11
					
										
			            		
    
		
										0
					
	
    

		
		
    
		
    
                                
                                                            
    
					
    Hello Kenneth, the 2nF is not critical - the circuit works with anything from 10pF upwards but using a somewhat larger Cap will reduce noise that feeds into the front end transistor.  However, that was not the question- my problem is that the the noise simulation will not converge with the ADA 4077 or similar but smoothly does so using the AD549 for the bottom amp.  Do you have any idea for that?  The answer to that question is not so important for my project; I am just beeing curious.  Thanks for taking the time to look into this!
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

    • 

	




Reply

    


  • 
	
    



	
		
	
	



    
	
    
		
    
							
					
    
		
    
		    
		    		    
			
				
									
							        
			
			
			
            
						
                            
            
            				
            
                on Jul 8, 2026 6:02 AM
            
            
												
						in reply to Kenneth11
					
										
			            		
    
		
										0
					
	
    

		
		
    
		
    
                                
                                                            
    
					
    Hello Kenneth, the 2nF is not critical - the circuit works with anything from 10pF upwards but using a somewhat larger Cap will reduce noise that feeds into the front end transistor.  However, that was not the question- my problem is that the the noise simulation will not converge with the ADA 4077 or similar but smoothly does so using the AD549 for the bottom amp.  Do you have any idea for that?  The answer to that question is not so important for my project; I am just beeing curious.  Thanks for taking the time to look into this!
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

    • 





Children
	
No Data


		
			








		

		

			
    
				
  • 
					
												
						
						
						
						
						
						
						

																										
						
						

					
					
					
				
    •