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OP42 Not Recommended for New Designs
The OP-42 is a fast precision JFET-input operational amplifier. Similar in speed to the OP-17, the OP-42 offers a symmetric 58V/µs slew rate and is internally...
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OP42 on Analog.com

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"hello world" circuit with op42

		
	
				

			

									
					    JMGL
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Sep 24, 2013 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
		    			
    
			    
    Hello guys,
    
 
    Sorry for this post, I am guessing it is extreamly basic but I just cannot get around it.
    
 
    I am using an op42, The connections are:
    
 
    1 (NULL) -> Not connected
    
 
    2 (-IN)     -> through 10k to (6) and through 10k to signal
    
 
    3 (+IN)    -> GROUND
    
 
    4 (V-)      -> GROUND
    
 
    5 (NULL) -> Not connected
    
 
    6 (OUT)  -> through 10k to (2) and output
    
 
    7 (V+)     -> ~5V
    
 
    8 (N.C.)  -> Not connected
    
 
    This is the simplest inverting cicuit that I could make. I was just playing with the op to learn how to use it. The signal is a square function from 0 to 1.5V.
    
 
    The output of that configuration, I was expecting to be the inverted signal, nevertheless, what I obtain is a square signal with really small pk to pk and with an offset of 1.5V.
    
 
    I have also tried with other resistors pairs (100R and 100R) and what I get is a DC voltage with the input signal on top.
    
 
    Searching the web, they recomended to use a compensating resitor (with resistance equal to the parallel equivalent of the other two resistors) between (3) and GROUND, but this did not change anything at all.
    
 
    Do you have any idea of what I am doing wrong?
    
 
    Thank you
    
 
    JMGL
    
			
    
			
									
		                             
                        

    
    
                                            
                 Operational Amplifiers (Op Amps)
                
                                                    
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                 op42
                
                                                    
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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Sep 24, 2013 5:46 AM
            
            
						
			            		
      
		
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      Hi Marcos, 
      
 
      The input voltage range of the OP42 looks like it can only accept voltages within 4V from the rails. Another thing is that since you're using an inverting configuration, you would expect your output to go from 0 to -1.5V. The output can't get there because your supplies are limited to ground. Can you possibly use dual supplies? This would solve both the input voltage range and the output problem. It should work with +/-5V supply. 
      
 
      If you can't provide a negative supply and you just want to learn how the op amp works, you could do a non-inverting configuration -- that is, have the same feedback circuit but tie one end of the 10k resistor to ground and apply the input to Vin+ instead, as in the figure below. You'll need to offset your input to about 3V for the amplifier to give you an output, and use a higher power supply, maybe about 10V. 
      
 
      
 
      Try this out and let me know if it works!
      
 
      Kris
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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    • 
	
      



	
			
		
	



      
	
      
		
      
							
					
      
		
      
		    
		    		    
			
				
									
							        
			
			
			
      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Sep 24, 2013 5:46 AM
            
            
						
			            		
      
		
										0
					
	
      

		
		
      
		
      
                                
                                                            
      
					
      Hi Marcos, 
      
 
      The input voltage range of the OP42 looks like it can only accept voltages within 4V from the rails. Another thing is that since you're using an inverting configuration, you would expect your output to go from 0 to -1.5V. The output can't get there because your supplies are limited to ground. Can you possibly use dual supplies? This would solve both the input voltage range and the output problem. It should work with +/-5V supply. 
      
 
      If you can't provide a negative supply and you just want to learn how the op amp works, you could do a non-inverting configuration -- that is, have the same feedback circuit but tie one end of the 10k resistor to ground and apply the input to Vin+ instead, as in the figure below. You'll need to offset your input to about 3V for the amplifier to give you an output, and use a higher power supply, maybe about 10V. 
      
 
      
 
      Try this out and let me know if it works!
      
 
      Kris
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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