------=_Part_123787_26610906.1156540915225 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Content-Disposition: inline Hi, The circuit I've attached is a staightforward op-amp amplifier with a gain of -10. For reasons outside the scope of this question*, I needed to add the resistor I've called Ra. In the traditional "virtual ground" method of analysis, Ra doesn't appear in the gain equation. I tried Ra=10k in my circuit, and indeed it continued to work unchanged. But I'm pretty sure if I set Ra=0 the circuit would stop working! To analyze this, I guess I will need a non-ideal op-amp model. What is the important parameter to include? Finite gain? Offset voltage? Regards, Mark markrages@gmail * Because someone will want to know, it's to limit the voltage swing at an analog switch IC (not pictured). -- You think that it is a secret, but it never has been one. - fortune cookie ------=_Part_123787_26610906.1156540915225 Content-Type: image/jpeg; name=circuit.jpg Content-Transfer-Encoding: base64 X-Attachment-Id: f_erb2a7id Content-Disposition: attachment; filename="circuit.jpg" /9j/4AAQSkZJRgABAQEASABIAAD/4QAWRXhpZgAATU0AKgAAAAgAAAAAAAD/2wBDABALDA4MChAO DQ4SERATGCgaGBYWGDEjJR0oOjM9PDkzODdASFxOQERXRTc4UG1RV19iZ2hnPk1xeXBkeFxlZ2P/ wAALCADVAdkBASIA/8QAGgABAQEBAQEBAAAAAAAAAAAAAQACBQQDBv/EAEYQAAEDAwIEAwUFBAcG BwEAAAEAAhEDBCEFMRJBUWFxgZEGEyKhsRQVMmLBQlLR4SMlcnOCsvAzQ1SDk6IkNERjwtLxkv/a AAgBAQAAPwD9cUhWypR4p5KG6pUrn0VzUpBCo6lQbmVpCVYQqUoGysKSpCVKhHNRVBChKIPVSD4K G2yjlZg8SI5qMo2GyvVZO6pjfdZPmrms7bIiSeiohZKjjdZKCIMbILZGeSP9FMdj6rpYVGVSroqJ SO6glChupSgqCFHzUFKxKVctkQpMKhUKjKoVHdREFUKhUIUVJGAiUK8EeCjlGxROFHaUDKDKyrxQ 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