Samuel Groner
Well-known member
Hi
For several applications (noise measurement, THD+N measurement, small signal testing etc.) I want to increase the vertical sensitivity of my scope with an external preamplifier; I set the following design goals:
* 100x gain
* very low input related voltage- and current-noise
* high input impedance, AC-coupled
* very low distortion
* excellent transient response
* 30 Vpp output swing
* input and ouput protected against overvoltage and shorts
And here's what I ended up with: [removed]
An exhaustive discussion of the design would be rather lengthy, so let me just point out a few interesting things:
* the balanced second stage provides very low offset (< 1 uV for matching parts), excellent current balance in the input pair and high open-loop gain (about 130 dB)
* the special biasing of Q2/Q3 provides some sort of common-mode feedback resulting in a simulated CMRR of better than 130 dB below 10 kHz (which is not likely to happen in real life though and rather unimportant for the given application anyway, but it doesn't hurt)
* the bootstrapped cascode Q6/Q7 of the input pair reduces input capacity in order to reduce distortion with high source impedances
* both small- and large-signal transient response is exeptionally good in simulation; the later shows a slew-rate of above 440 V/us
Now a few questions:
* Do you think the drift performance of the amplifier is good enough to skip the DC servo? I'd aim for < 10 mV output offset.
* The drains of the input pair is biased at 5.3 V; is this a well-chosen voltage? I'd have thought that a tad lower wouldn't hurt but SPICE indicated that it won't work.
* Do you think that it is promising to experiment with a three-stage architecture (i.e. as shown in the LT1001 datasheet) in order to improve distortion and precision even further (and probably loose some of the bandwith and transient performance)?
* While simulating I got a lot of "Gmin step failure", "Gmin stepping failure" and "source stepping failed" errors--especially after introducing the bootstrapped cascode, paralleled output devices or DC servo; any hints how to get rid of them?
Thanks for your comments and suggestions!
Samuel
For several applications (noise measurement, THD+N measurement, small signal testing etc.) I want to increase the vertical sensitivity of my scope with an external preamplifier; I set the following design goals:
* 100x gain
* very low input related voltage- and current-noise
* high input impedance, AC-coupled
* very low distortion
* excellent transient response
* 30 Vpp output swing
* input and ouput protected against overvoltage and shorts
And here's what I ended up with: [removed]
An exhaustive discussion of the design would be rather lengthy, so let me just point out a few interesting things:
* the balanced second stage provides very low offset (< 1 uV for matching parts), excellent current balance in the input pair and high open-loop gain (about 130 dB)
* the special biasing of Q2/Q3 provides some sort of common-mode feedback resulting in a simulated CMRR of better than 130 dB below 10 kHz (which is not likely to happen in real life though and rather unimportant for the given application anyway, but it doesn't hurt)
* the bootstrapped cascode Q6/Q7 of the input pair reduces input capacity in order to reduce distortion with high source impedances
* both small- and large-signal transient response is exeptionally good in simulation; the later shows a slew-rate of above 440 V/us
Now a few questions:
* Do you think the drift performance of the amplifier is good enough to skip the DC servo? I'd aim for < 10 mV output offset.
* The drains of the input pair is biased at 5.3 V; is this a well-chosen voltage? I'd have thought that a tad lower wouldn't hurt but SPICE indicated that it won't work.
* Do you think that it is promising to experiment with a three-stage architecture (i.e. as shown in the LT1001 datasheet) in order to improve distortion and precision even further (and probably loose some of the bandwith and transient performance)?
* While simulating I got a lot of "Gmin step failure", "Gmin stepping failure" and "source stepping failed" errors--especially after introducing the bootstrapped cascode, paralleled output devices or DC servo; any hints how to get rid of them?
Thanks for your comments and suggestions!
Samuel