Dear Dennis... analogZONE's Dennis Feucht answers your design queries in his new Circuit Design Clinic! February 2007

analogZONE presents an interactive analog design service to readers! Send us your design questions (with relevant data; schematics in JPEG or GIF, please) for some free engineering advice from analogZONE's circuit consultant, Dennis Feucht, on how you might solve a design problem or improve circuit performance. Submissions may be edited for clarity or brevity, and submitters and their email addresses will remain anonymous (unless otherwise indicated). Please send your questions to Dennis here.

What's Wrong With This Circuit?

This circuit design clinic is reminiscent of those visual amusements for children asking them "What's wrong with this picture?" The exercise is to find as many errors as possible, in this case, of a given circuit design.

Instead of subjecting the circuits of others to critique, or even ridicule, the following circuit diagram is most of the vertical amplifier of the first oscilloscope I ever designed (in my late teens) for my own use. As an amplifier, this circuit exercise applies to a wide range of analog circuit applications, not only to 'scopes. I needed a calibrated 'scope for my home laboratory, and got a good deal on a used Knight-kit oscilloscope, which gave me the chassis, power supply (especially the CRT high-voltage supply), CRT circuit, and front-panel. I gutted the rest and redesigned what became my first laboratory-quality oscilloscope.

Needless to say (as you will see), my design skills were in their early developmental stage, and this design provides plenty of opportunity to find various errors in the form of suboptimal design decisions. Find as many as you can without changing the basic circuit topology.

Read the entire response here (44 kb Adobe Acrobat PDF file)