Discrete Opamp Open Design

[RNMarsh]

Originally Posted by scott wurcer

and at 8mA bias and 600 Ohm load still only had .01% distortion.

The Sziklai pair has feedback and in my experience is much more prone to oscillate, but the added gain typically reduces the distortion by a larger factor.

I love that bootstrapped diamond output stage of your's. It's an instant classic.

I tried the Sziklai pair applied a little differently.... Most of the time the first transistor has at least a 100 ohm or greater resistor in its collector for local gain. This can lead to unstable operation. Instead, I used the pair just as compliments in an attempt to lower Output Stage distortion. Low gain so it is very stable (from just a 15 Ohm sampling R in the collector). Each Sziklai is made with matched compliments and have same emitter resistors and same currents thru each. Then run two pair in push-pull to any level needed (class A or AB). It just makes a super linear emitter-follower. Seems to work very well. Thx-RNMarsh

[luvdunhill]

Originally Posted by bcarso

Look at signal-to-noise at a given frequency to determine how sensitive it will be to impedances, and then make the determination about how the filter components will load the amp output. Then fold in the loading thereafter.

Every situation is different, and I know of no rule-of-thumb. Lower Z is always lower noise, but depending on where the circuit is in the system you may be well-dominated by noise in the input signal already, so there may be no reason for heroic measures in the filter.

To be clear, SNR versus frequency for the opamp? Like this? One is at gain of 10 and other is configured as a buffer.

I'm assuming a pre->buffer->opamp->opamp->buffer->amp config.

[bcarso]

Originally Posted by luvdunhill

To be clear, SNR versus frequency for the opamp? Like this? One is at gain of 10 and other is configured as a buffer.

I'm assuming a pre->buffer->opamp->opamp->buffer->amp config.

So you are cascading two Sallen-Key filter sections with buffer ahead and after? Are you defining a large passband in this fashion, thus a low-frequency highpass followed by a high-frequency lowpass?

Circuits using opamps, and in particular filters, have "noise gain", and Sallen-Key topologies are no exception. Actually some S-K ones are quite low noise gain compared to other topologies. In any event, lower impedances (smaller resistances, bigger capacitances) to realize a given transfer function will always be lower noise (lower thermal noise due to the resistors), but will require more current to drive, both from a given driving source and from the opamp itself.

The noise gain, meaning the frequency-dependent multiplier of the opamp voltage noise (the opamp input current noise effects are treated separately), is dependent on, but not identical to, the realized overall filter transfer function. Higher Q will generally entail higher noise gain. For most audio work with lowpass and highpass filters, the Q will be modest (little or no peaking in the frequency domain transitioning into the stopband).

Off topic (you may stop reading now):

Boy it's nice to be done with jury duty. Six days, and at the end hopelessly deadlocked. Wouldn't you know two of the three for acquittal had engineering backgrounds

[scott wurcer]

Originally Posted by bcarso

My experience as well with Sziklai. And the poorer ability to turn off an output device translates into a dynamic asymmetry that is somewhat bothersome at high frequencies. Bryston has used a triple compound that they have managed to make work, but it looks very temperamental in simulation.

I used it in one of my xDSL drivers, it worked too well and the lowest of the 4 bias settings met all the specs, 400mA peak at 250uA bias and still maintaining -65dbc at 2MHz. Pain to stabilize though. It was too bad the lifetime was short because it was true differential and everyone decided they wanted plain dual op-amps (multi-source) even though the application suffers from common-mode oscillation/peaking issues.

[luvdunhill]

Originally Posted by bcarso

So you are cascading two Sallen-Key filter sections with buffer ahead and after? Are you defining a large passband in this fashion, thus a low-frequency highpass followed by a high-frequency lowpass?

that, or two LPF or two HPF, etc.

Thanks for the explanation. It would still be nice to have a rule of thumb. It sounds like the answer is to construct a model for the entire filter and evaluate it en masse. Perhaps, stepping through the model over multiple xover frequencies. Drop in another opamp model and repeat, rinse, etc.

[dimitri]

This might be related to Sallen-Key LPF filter design

[bcarso]

Originally Posted by dimitri

This might be related to Sallen-Key LPF filter design

Nice ref.

[scott wurcer]

Originally Posted by bcarso

Nice ref.

It does contain the most common mis-conception about CFA's. I rewrote Black's basic equation for VFA's and CFA's to create equivalent terms and it is clear that the CURRENT into the inverting input is the feedback signal.

Or take the simplest case, a follower, in a VFA and instantaneous step appears at the input and the voltage across the inputs determines the current delivered to the gain node, in a CFA the instantaneous current is determined by the output voltage across the feedback resistor or the current into the inverting input.

[luvdunhill]

woah, ok. Could someone distill down Table 2 to a simple LTSpice circuit I can treat as a load?

I took my guess above from this:

https://www.circuitlab.com/circuit/t...put-impedance/

[grhughes]

Scott: How is the breadboard build along? What parts do you want to change? Did you use the 50 ohm pot between the diff amp? I won't proceed with mine until you freeze the design. Ray