In keeping with the subject of jellybeat BJT tranny's - Page 1
Forum section: electronics / projects

author: asgard
author_rank: Regular Contributor
timestamp: February 18, 2024, 11:15:40 pm
content: ...

author: Benta
author_rank: Super Contributor
timestamp: February 18, 2024, 11:29:32 pm
content: OK.Interesting approach using PNP transistors with a positive VCC (I assume that's what the little "+" means, and you mention 3.3 V as well).Let us know how that works out for you.

author: temperance
author_rank: Super Contributor
timestamp: February 19, 2024, 12:23:30 am
content: I don't see how this can ever work.

author: asgard
author_rank: Regular Contributor
timestamp: February 19, 2024, 01:02:03 am
content: Yes I know spice sims are not perfect, but I threw a whole lot of different transients at it to characterize the rough response and found it "good enough".

author: temperance
author_rank: Super Contributor
timestamp: February 19, 2024, 01:04:29 am
content: Can you compare the first schematic you've posted with the one on the spice simulation?

author: asgard
author_rank: Regular Contributor
timestamp: February 19, 2024, 01:20:47 am
content: Per the remarks about top 5 jellybean BJT's here is an application I use for one of them. This is a Schmitt trigger gate front-end for a synth module that is meant to accomplish these goals:1. Be able to accept gate or trigger inputs minimizing issues due to hysteresis.2. Be robust with respect to the voltage inputs, not allowing the signals downstream to exceed 3.3V, or do anything weird if the input is either shorted or fed negative voltages.3. Provide at least 100K input impedance.For this circuit the VCC voltage source is 3.3V, and the packaging for the trannys are dual devices in the SOT50P170X60-6N package. This helps reduce the footprint of the whole device.

author: temperance
author_rank: Super Contributor
timestamp: February 19, 2024, 03:27:10 am
content: Much better.QuoteThis circuit doesn't require matched transistors. Dual transistors will only save space and assembly cost.LT spice does not simulate reverse BE bias breakdown. As such you might want to replace D6 with a zener diode + a diode in series to protect the first transistor from reverse BE bias. (Typ. max 6 V)For ESD protection a 1...10 nF capacitor from the first transistor base to 0 V is also recommended.

author: asgard
author_rank: Regular Contributor
timestamp: February 19, 2024, 07:29:39 pm
content: So what you are saying is something in the line of this? I suppose the forward drop of .7V or so ought not to interfere with the Schmitt action all that much. I'll need to rework the sim to confirm this.

author: temperance
author_rank: Super Contributor
timestamp: February 19, 2024, 08:18:15 pm
content: My mistake. It doesn't require two diodes. One zener is enough and D9 can be removed.

author: asgard
author_rank: Regular Contributor
timestamp: February 21, 2024, 06:01:29 am
content: This is the digital out side, a pair of transistor inverters arranged as a buffer/amplifier raising the 3.3V output digital signal to a +5V level. There is a pair of analog input/output circuits. The last analog piece shows how I generate VREF and analog rails from a eurorack-style power supply.

author: temperance
author_rank: Super Contributor
timestamp: February 25, 2024, 01:03:00 pm
content: Those look fine too me. The ferrite beads on the power supply might not be necessary and they can cause unwanted resonances if the capacitors following the ferrite beads are high Q capacitors like MLCC's. A low pass filter made up of a low value resistor (1...10 R) in series with the power supply input can be a more effective low pass filter if a low value resistor can be used.https://www.analog.com/en/resources/analog-dialogue/articles/ferrite-beads-demystified.html