25.11.2005, 04:47 PM
Okay, I'm still hard at work with the LTspice simulator and getting better at it every day 
Due to the fact that my semi-discrete driver-stage of my previous design suffered from a excessive power-drain I decided to try the readily available TDA8939 and work in some of the things I learned this week as well.
The first design I tried (why start easy no?) is a full-bridge UcD with some extra bits thrown in. So why no more SODFA? Well, I learned a few things from Beobachters designs posted here and drew my own conclusions. One of the problems with UcD is that it is hard to precisely control the carrier frequency. It is basically controlled by the delay between the in- and output. Below is the principle schematic for a UcD amplifier courtesy of the talented Bruno Putzeys.
Bruno's UcD concept
[Bild: 11696.gif]
Obviously in the Hypex UcD modules the comparator and MOSFET drivers are exactly matched so that the carrier freqeuncy can be controlled. This however is a balancing act which is hard to follow for the DIY-er, especially if you want to use ICs instead of discrete components. There's a way around this though, as you can simply include a R/C network with the UcD concept and connect that to the comparator's other input. Now you have control over the carrier frequency. Unfortunately doing it that way will shift the hysterisis of the comparator and hence introduce non-linear distortion, so we are faced with a whole new problem.
UcD with R/C network
[Bild: 11697.gif]
Fortunately that is a problem that can easily be tackled by using a 2nd comparator, see below, and put the R/C network at its input. Hence there's no shift in hysterisis and the carrier frequency can be controlled exactly.
Improved UcD concept
[Bild: 11698.gif]
The big advantage obviously is that you can use fast opamps and comparators that are readily available, such as the LT1016 used in the above pictured UcD design without having to exactly match propagation delay in a design to reach a certain target carrier frequency. It can now be simply controlled by adding a second (ultra-fast if you like) comparator with a simple R/C network.
The half-bridge and full-bridge design using the TDA8939 both use this R/C network as the TDA itself is a power-comparator and I thus did not need to add another component, I just needed to insert the R/C network. Below you'll find a screenshot of the full-bridge TDA8939 using this principe, which, surprise! surprise! doesn't seem to work fully yet. I'll post the ASC, THD and efficiency report in a separate post, lets first look at the 2nd design.
tda8939_ucd_bridged_001
[Bild: 11701.gif]
The 2nd design is the same concept, but I just dropped the full-bridge and wired the amplifier as a half bridge. Only one channel is connected and plotted, although two channels are driven in the schematic. That isn't important though, what counts is that the idea works, you just need to duplicate the circuit around the LT1016 to build a stereo amp. What it shows is though that a TDA8939 is a viable candidate for a half-bridge UcD amplifier as shown below.
tda8939_ucd_002
[Bild: 11702.gif]
Carrier-frequency is a bit on the low side but that can all be adjusted with the R/C network that is visible at both the normal and inverted comparator output. What I've shown here however, and what Beobachter failed to do, that complex designs with high-speed components are possible, it just takes a bit of effort and cleverness.
As always I'm looking forward to all of your replies!
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
Due to the fact that my semi-discrete driver-stage of my previous design suffered from a excessive power-drain I decided to try the readily available TDA8939 and work in some of the things I learned this week as well.The first design I tried (why start easy no?) is a full-bridge UcD with some extra bits thrown in. So why no more SODFA? Well, I learned a few things from Beobachters designs posted here and drew my own conclusions. One of the problems with UcD is that it is hard to precisely control the carrier frequency. It is basically controlled by the delay between the in- and output. Below is the principle schematic for a UcD amplifier courtesy of the talented Bruno Putzeys.
Bruno's UcD concept
[Bild: 11696.gif]
Obviously in the Hypex UcD modules the comparator and MOSFET drivers are exactly matched so that the carrier freqeuncy can be controlled. This however is a balancing act which is hard to follow for the DIY-er, especially if you want to use ICs instead of discrete components. There's a way around this though, as you can simply include a R/C network with the UcD concept and connect that to the comparator's other input. Now you have control over the carrier frequency. Unfortunately doing it that way will shift the hysterisis of the comparator and hence introduce non-linear distortion, so we are faced with a whole new problem.
UcD with R/C network
[Bild: 11697.gif]
Fortunately that is a problem that can easily be tackled by using a 2nd comparator, see below, and put the R/C network at its input. Hence there's no shift in hysterisis and the carrier frequency can be controlled exactly.
Improved UcD concept
[Bild: 11698.gif]
The big advantage obviously is that you can use fast opamps and comparators that are readily available, such as the LT1016 used in the above pictured UcD design without having to exactly match propagation delay in a design to reach a certain target carrier frequency. It can now be simply controlled by adding a second (ultra-fast if you like) comparator with a simple R/C network.
The half-bridge and full-bridge design using the TDA8939 both use this R/C network as the TDA itself is a power-comparator and I thus did not need to add another component, I just needed to insert the R/C network. Below you'll find a screenshot of the full-bridge TDA8939 using this principe, which, surprise! surprise! doesn't seem to work fully yet. I'll post the ASC, THD and efficiency report in a separate post, lets first look at the 2nd design.
tda8939_ucd_bridged_001
[Bild: 11701.gif]
The 2nd design is the same concept, but I just dropped the full-bridge and wired the amplifier as a half bridge. Only one channel is connected and plotted, although two channels are driven in the schematic. That isn't important though, what counts is that the idea works, you just need to duplicate the circuit around the LT1016 to build a stereo amp. What it shows is though that a TDA8939 is a viable candidate for a half-bridge UcD amplifier as shown below.
tda8939_ucd_002
[Bild: 11702.gif]
Carrier-frequency is a bit on the low side but that can all be adjusted with the R/C network that is visible at both the normal and inverted comparator output. What I've shown here however, and what Beobachter failed to do, that complex designs with high-speed components are possible, it just takes a bit of effort and cleverness.
As always I'm looking forward to all of your replies!
Best regards,
Sander Sassen
http://www.hardwareanalysis.com