Designing an RF amplifier. Concept is, high power, complementary cascodes for the output stage (with heavy class A use, but being PP, class AB is an option). 50 ohm output, direct drive, say 10W level. I happen to have a complementary pair that's not too slow (2SC2690A and 2SA1220A), and I'd like to maximize the bandwidth around that. The NPN side is fine, I have a 30V, 1A, 2GHz transistor that would pair very well with it. Don't have any such thing for the PNP side. So, what if I fake a PNP, by wrapping, say, a BFT92 around the NPN? http://seventransistorlabs.com/Images/Sziklai_Cascode.png That'd be Q1 = BFT92, Q2 = 2SC4821, Q3 = 2SA1220A, and resistors for flavor, but probably roughly representative. (Ground wouldn't actually be ground-ground, but probably something like +40V, and "+12V" would be +45V.) The combination is still fast... ah, but Sziklai connections have a propensity for oscillation all their own, let alone in a cascode, plus whatever other machinations I might have for feedback around the thing. Game killer? The other option would be folded cascode, which is understandably rather wasteful for a power stage! Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Cascode substitutions
Started by ●January 31, 2016
Reply by ●January 31, 20162016-01-31
On 1/31/2016 6:34 AM, Tim Williams wrote:> Designing an RF amplifier. Concept is, high power, complementary cascodes > for the output stage (with heavy class A use, but being PP, class AB is an > option). 50 ohm output, direct drive, say 10W level. > > I happen to have a complementary pair that's not too slow (2SC2690A and > 2SA1220A), and I'd like to maximize the bandwidth around that. The NPN > side is fine, I have a 30V, 1A, 2GHz transistor that would pair very well > with it. Don't have any such thing for the PNP side. > > So, what if I fake a PNP, by wrapping, say, a BFT92 around the NPN? > http://seventransistorlabs.com/Images/Sziklai_Cascode.png > That'd be Q1 = BFT92, Q2 = 2SC4821, Q3 = 2SA1220A, and resistors for > flavor, but probably roughly representative. (Ground wouldn't actually be > ground-ground, but probably something like +40V, and "+12V" would be > +45V.) > > The combination is still fast... ah, but Sziklai connections have a > propensity for oscillation all their own, let alone in a cascode, plus > whatever other machinations I might have for feedback around the thing. > Game killer? > > The other option would be folded cascode, which is understandably rather > wasteful for a power stage!Simple is usually better - it's easier to get unconditional stability when the PZ count is small, especially if you're contemplating feedback. Cascode helps with Miller capacitance but it introduces addition phase shift in the forward and reverse path - feedback becomes less attractive. -- Grizzly H.
Reply by ●January 31, 20162016-01-31
On 01/31/2016 07:57 AM, mixed nuts wrote:> On 1/31/2016 6:34 AM, Tim Williams wrote: >> Designing an RF amplifier. Concept is, high power, complementary >> cascodes >> for the output stage (with heavy class A use, but being PP, class AB >> is an >> option). 50 ohm output, direct drive, say 10W level. >> >> I happen to have a complementary pair that's not too slow (2SC2690A and >> 2SA1220A), and I'd like to maximize the bandwidth around that. The NPN >> side is fine, I have a 30V, 1A, 2GHz transistor that would pair very well >> with it. Don't have any such thing for the PNP side. >> >> So, what if I fake a PNP, by wrapping, say, a BFT92 around the NPN? >> http://seventransistorlabs.com/Images/Sziklai_Cascode.png >> That'd be Q1 = BFT92, Q2 = 2SC4821, Q3 = 2SA1220A, and resistors for >> flavor, but probably roughly representative. (Ground wouldn't >> actually be >> ground-ground, but probably something like +40V, and "+12V" would be >> +45V.) >> >> The combination is still fast... ah, but Sziklai connections have a >> propensity for oscillation all their own, let alone in a cascode, plus >> whatever other machinations I might have for feedback around the thing. >> Game killer? >> >> The other option would be folded cascode, which is understandably rather >> wasteful for a power stage! > > Simple is usually better - it's easier to get unconditional stability > when the PZ count is small, especially if you're contemplating feedback. > Cascode helps with Miller capacitance but it introduces addition phase > shift in the forward and reverse path - feedback becomes less attractive. >I'll register a contrarian view here. Sziklai pairs and other forms of local feedback can improve linearity out of all recognition. Of course RF PAs are more vulnerable to stray inductance than the small signal stuff I generally do, but if you model that carefully you might make a real winner. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●January 31, 20162016-01-31
"Tim Williams" <tiwill@seventransistorlabs.com> Wrote in message:> Designing an RF amplifier. Concept is, high power, complementary cascodes > for the output stage (with heavy class A use, but being PP, class AB is an > option). 50 ohm output, direct drive, say 10W level. > > I happen to have a complementary pair that's not too slow (2SC2690A and > 2SA1220A), and I'd like to maximize the bandwidth around that. The NPN > side is fine, I have a 30V, 1A, 2GHz transistor that would pair very well > with it. Don't have any such thing for the PNP side. > > So, what if I fake a PNP, by wrapping, say, a BFT92 around the NPN? > http://seventransistorlabs.com/Images/Sziklai_Cascode.png > That'd be Q1 = BFT92, Q2 = 2SC4821, Q3 = 2SA1220A, and resistors for > flavor, but probably roughly representative. (Ground wouldn't actually be > ground-ground, but probably something like +40V, and "+12V" would be > +45V.) > > The combination is still fast... ah, but Sziklai connections have a > propensity for oscillation all their own, let alone in a cascode, plus > whatever other machinations I might have for feedback around the thing. > Game killer? > > The other option would be folded cascode, which is understandably rather > wasteful for a power stage! > > Tim > > -- > Seven Transistor Labs, LLC > Electrical Engineering Consultation and Contract Design > Website: http://seventransistorlabs.com > > >Not sure how useful this is to your specific probelm, but there are some interesting topologies in this patent application, particularly figures 5, 6, and 7. https://docs.google.com/viewer?url=patentimages.storage.googleapis .com/pdfs/US3163827.pdf -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Reply by ●January 31, 20162016-01-31
On Sun, 31 Jan 2016 05:34:23 -0600, "Tim Williams" <tiwill@seventransistorlabs.com> wrote:>Designing an RF amplifier. Concept is, high power, complementary cascodes >for the output stage (with heavy class A use, but being PP, class AB is an >option). 50 ohm output, direct drive, say 10W level. > >I happen to have a complementary pair that's not too slow (2SC2690A and >2SA1220A), and I'd like to maximize the bandwidth around that. The NPN >side is fine, I have a 30V, 1A, 2GHz transistor that would pair very well >with it. Don't have any such thing for the PNP side. > >So, what if I fake a PNP, by wrapping, say, a BFT92 around the NPN? >http://seventransistorlabs.com/Images/Sziklai_Cascode.png >That'd be Q1 = BFT92, Q2 = 2SC4821, Q3 = 2SA1220A, and resistors for >flavor, but probably roughly representative. (Ground wouldn't actually be >ground-ground, but probably something like +40V, and "+12V" would be >+45V.) > >The combination is still fast... ah, but Sziklai connections have a >propensity for oscillation all their own, let alone in a cascode, plus >whatever other machinations I might have for feedback around the thing. >Game killer? > >The other option would be folded cascode, which is understandably rather >wasteful for a power stage! > >TimWhat's the frequency and the bandwidth requirement? Regular low-frequency circuit topologies generally don't scale well into RF amps. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●January 31, 20162016-01-31
John Larkin <jjlarkin@highlandtechnology.com> Wrote in message:> On Sun, 31 Jan 2016 05:34:23 -0600, "Tim Williams" > <tiwill@seventransistorlabs.com> wrote: > >>Designing an RF amplifier. Concept is, high power, complementary cascodes >>for the output stage (with heavy class A use, but being PP, class AB is an >>option). 50 ohm output, direct drive, say 10W level. >> >>I happen to have a complementary pair that's not too slow (2SC2690A and >>2SA1220A), and I'd like to maximize the bandwidth around that. The NPN >>side is fine, I have a 30V, 1A, 2GHz transistor that would pair very well >>with it. Don't have any such thing for the PNP side. >> >>So, what if I fake a PNP, by wrapping, say, a BFT92 around the NPN? >>http://seventransistorlabs.com/Images/Sziklai_Cascode.png >>That'd be Q1 = BFT92, Q2 = 2SC4821, Q3 = 2SA1220A, and resistors for >>flavor, but probably roughly representative. (Ground wouldn't actually be >>ground-ground, but probably something like +40V, and "+12V" would be >>+45V.) >> >>The combination is still fast... ah, but Sziklai connections have a >>propensity for oscillation all their own, let alone in a cascode, plus >>whatever other machinations I might have for feedback around the thing. >>Game killer? >> >>The other option would be folded cascode, which is understandably rather >>wasteful for a power stage! >> >>Tim > > What's the frequency and the bandwidth requirement? > > Regular low-frequency circuit topologies generally don't scale well > into RF amps. >Does anyone use that admittance/Z parameter matrix stuff for RF amp design/stability analysis that I've read about in books like "Intro to RF Design" by Wes Hayward, or do they just mess with it in Spice and are then like "eh works well enough ship it" It seems difficult to even find device data sheets that have those parameters evaluated at enough data points to be useful (so if you don't have a 20k network analyzer to check it yourself at the frequency of interest you're SOL), and I once searched for software that could deal with such things that didn't cost 5k and I couldn't really find much. -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Reply by ●January 31, 20162016-01-31
On 01/31/2016 12:26 PM, bitrex wrote:> John Larkin <jjlarkin@highlandtechnology.com> Wrote in message: >> On Sun, 31 Jan 2016 05:34:23 -0600, "Tim Williams" >> <tiwill@seventransistorlabs.com> wrote: >> >>> Designing an RF amplifier. Concept is, high power, complementary cascodes >>> for the output stage (with heavy class A use, but being PP, class AB is an >>> option). 50 ohm output, direct drive, say 10W level. >>> >>> I happen to have a complementary pair that's not too slow (2SC2690A and >>> 2SA1220A), and I'd like to maximize the bandwidth around that. The NPN >>> side is fine, I have a 30V, 1A, 2GHz transistor that would pair very well >>> with it. Don't have any such thing for the PNP side. >>> >>> So, what if I fake a PNP, by wrapping, say, a BFT92 around the NPN? >>> http://seventransistorlabs.com/Images/Sziklai_Cascode.png >>> That'd be Q1 = BFT92, Q2 = 2SC4821, Q3 = 2SA1220A, and resistors for >>> flavor, but probably roughly representative. (Ground wouldn't actually be >>> ground-ground, but probably something like +40V, and "+12V" would be >>> +45V.) >>> >>> The combination is still fast... ah, but Sziklai connections have a >>> propensity for oscillation all their own, let alone in a cascode, plus >>> whatever other machinations I might have for feedback around the thing. >>> Game killer? >>> >>> The other option would be folded cascode, which is understandably rather >>> wasteful for a power stage! >>> >>> Tim >> >> What's the frequency and the bandwidth requirement? >> >> Regular low-frequency circuit topologies generally don't scale well >> into RF amps. >> > > > Does anyone use that admittance/Z parameter matrix stuff for RF > amp design/stability analysis that I've read about in books like > "Intro to RF Design" by Wes Hayward, or do they just mess with it > in Spice and are then like "eh works well enough ship > it" > > It seems difficult to even find device data sheets that have those > parameters evaluated at enough data points to be useful (so if > you don't have a 20k network analyzer to check it yourself at the > frequency of interest you're SOL), and I once searched for > software that could deal with such things that didn't cost 5k and > I couldn't really find much. >The classical approach has the great virtue that it tells you how well you _can_ do, so you don't wind up attempting recreational impossibilities on the one hand, or turning a silk purse back into a sow's ear on the other. Cheers Phil "Do the math" Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●January 31, 20162016-01-31
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> Wrote in message:> On 01/31/2016 12:26 PM, bitrex wrote: >> John Larkin <jjlarkin@highlandtechnology.com> Wrote in message: >>> On Sun, 31 Jan 2016 05:34:23 -0600, "Tim Williams" >>> <tiwill@seventransistorlabs.com> wrote: >>> >>>> Designing an RF amplifier. Concept is, high power, complementary cascodes >>>> for the output stage (with heavy class A use, but being PP, class AB is an >>>> option). 50 ohm output, direct drive, say 10W level. >>>> >>>> I happen to have a complementary pair that's not too slow (2SC2690A and >>>> 2SA1220A), and I'd like to maximize the bandwidth around that. The NPN >>>> side is fine, I have a 30V, 1A, 2GHz transistor that would pair very well >>>> with it. Don't have any such thing for the PNP side. >>>> >>>> So, what if I fake a PNP, by wrapping, say, a BFT92 around the NPN? >>>> http://seventransistorlabs.com/Images/Sziklai_Cascode.png >>>> That'd be Q1 = BFT92, Q2 = 2SC4821, Q3 = 2SA1220A, and resistors for >>>> flavor, but probably roughly representative. (Ground wouldn't actually be >>>> ground-ground, but probably something like +40V, and "+12V" would be >>>> +45V.) >>>> >>>> The combination is still fast... ah, but Sziklai connections have a >>>> propensity for oscillation all their own, let alone in a cascode, plus >>>> whatever other machinations I might have for feedback around the thing. >>>> Game killer? >>>> >>>> The other option would be folded cascode, which is understandably rather >>>> wasteful for a power stage! >>>> >>>> Tim >>> >>> What's the frequency and the bandwidth requirement? >>> >>> Regular low-frequency circuit topologies generally don't scale well >>> into RF amps. >>> >> >> >> Does anyone use that admittance/Z parameter matrix stuff for RF >> amp design/stability analysis that I've read about in books like >> "Intro to RF Design" by Wes Hayward, or do they just mess with it >> in Spice and are then like "eh works well enough ship >> it" >> >> It seems difficult to even find device data sheets that have those >> parameters evaluated at enough data points to be useful (so if >> you don't have a 20k network analyzer to check it yourself at the >> frequency of interest you're SOL), and I once searched for >> software that could deal with such things that didn't cost 5k and >> I couldn't really find much. >> > > The classical approach has the great virtue that it tells you how well > you _can_ do, so you don't wind up attempting recreational > impossibilities on the one hand, or turning a silk purse back into a > sow's ear on the other. > > Cheers > > Phil "Do the math" Hobbs >That approach cetainly is appealing, but it seems like there often just aren't the tools available to do it. Do I have to do all the impedande/admittance matrix inversions myself, or write my own software? If the required data is even available? This is 2k16, come on! I have Angry Birds waiting on me... -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Reply by ●January 31, 20162016-01-31
On 1/31/2016 12:26 PM, bitrex wrote:> John Larkin <jjlarkin@highlandtechnology.com> Wrote in message: >> On Sun, 31 Jan 2016 05:34:23 -0600, "Tim Williams" >> <tiwill@seventransistorlabs.com> wrote: >> >>> Designing an RF amplifier. Concept is, high power, complementary cascodes >>> for the output stage (with heavy class A use, but being PP, class AB is an >>> option). 50 ohm output, direct drive, say 10W level. >>> >>> I happen to have a complementary pair that's not too slow (2SC2690A and >>> 2SA1220A), and I'd like to maximize the bandwidth around that. The NPN >>> side is fine, I have a 30V, 1A, 2GHz transistor that would pair very well >>> with it. Don't have any such thing for the PNP side. >>> >>> So, what if I fake a PNP, by wrapping, say, a BFT92 around the NPN? >>> http://seventransistorlabs.com/Images/Sziklai_Cascode.png >>> That'd be Q1 = BFT92, Q2 = 2SC4821, Q3 = 2SA1220A, and resistors for >>> flavor, but probably roughly representative. (Ground wouldn't actually be >>> ground-ground, but probably something like +40V, and "+12V" would be >>> +45V.) >>> >>> The combination is still fast... ah, but Sziklai connections have a >>> propensity for oscillation all their own, let alone in a cascode, plus >>> whatever other machinations I might have for feedback around the thing. >>> Game killer? >>> >>> The other option would be folded cascode, which is understandably rather >>> wasteful for a power stage! >>> >>> Tim >> >> What's the frequency and the bandwidth requirement? >> >> Regular low-frequency circuit topologies generally don't scale well >> into RF amps. >> > > Does anyone use that admittance/Z parameter matrix stuff for RF > amp design/stability analysis that I've read about in books like > "Intro to RF Design" by Wes Hayward, or do they just mess with it > in Spice and are then like "eh works well enough ship > it"Not so much anymore - it's all wired into RF design packages - some bloody expensive, a couple free. SPICE input files can be used - if you have a well characterized SPICE model, the simulation engine will understand it. S-Parameter simulations are the norm. Generally, these packages will work with MATLAB/Octave, VHDL, Verilog... or have something similar built in - lots of post processing and modeling options. SPICE sucks for modeling transmission lines. The RF/Microwave packages will work directly with dimensions and materials - just plug in the numbers or click on a library definition. Life is good. Optimizers are generally available - select the free parameters, set up goals and constraints, go for coffee. Bandpass, gain, linearity, noise figure... can be optimized and often a layout is only a few clicks away. Transient analysis can be done same as SPICE - current and voltage waveforms are available with some limitations. If you have a device defined with S-Parameters, the non-linear properties aren't available for the simulation - you'll need a circuit or behavioral model for that. There's a "Harmonic Balance" solver in RF/Microwave packages (google Spectre RF - Ken Kundert http://kenkundert.com/ ) that is used for distortion and intermodulation analysis - function similar to transient analysis but done in the frequency domain with an FFT in the solution loop to achieve much higher accuracy and faster convergence. The books are important however - it's important to understand what you're doing - RF design is not the same as boiling coffee water.> > It seems difficult to even find device data sheets that have those > parameters evaluated at enough data points to be useful (so if > you don't have a 20k network analyzer to check it yourself at the > frequency of interest you're SOL), and I once searched for > software that could deal with such things that didn't cost 5k and > I couldn't really find much.Look at QUCS - GPLed Design package if you want to get your feet wet. No layout capability but otherwise very functional. SPICE models can be added but the usual issues encountered with SPICE variants apply - PSPICE is not the same as the SPICE 3 derivatives, encrypted stuff isn't supported, you'll sometimes have to go through converting a netlist to a subcircuit schematic to get an opamp to work right etc. https://en.wikipedia.org/wiki/Quite_Universal_Circuit_Simulator http://qucs.sourceforge.net/ If you decide to try it, install Octave (4.0.0 is current) first. The windows port works but has some annoying issues that are probably related to specific device drivers - print formatting, display quirks, access rights etc. If you use Linux, there are tested binaries in the repositories for most distros which are generally very clean. -- Grizzly H.
Reply by ●January 31, 20162016-01-31
On 01/31/2016 02:36 PM, bitrex wrote:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> Wrote in message: >> On 01/31/2016 12:26 PM, bitrex wrote: >>> John Larkin <jjlarkin@highlandtechnology.com> Wrote in message: >>>> On Sun, 31 Jan 2016 05:34:23 -0600, "Tim Williams" >>>> <tiwill@seventransistorlabs.com> wrote: >>>> >>>>> Designing an RF amplifier. Concept is, high power, complementary cascodes >>>>> for the output stage (with heavy class A use, but being PP, class AB is an >>>>> option). 50 ohm output, direct drive, say 10W level. >>>>> >>>>> I happen to have a complementary pair that's not too slow (2SC2690A and >>>>> 2SA1220A), and I'd like to maximize the bandwidth around that. The NPN >>>>> side is fine, I have a 30V, 1A, 2GHz transistor that would pair very well >>>>> with it. Don't have any such thing for the PNP side. >>>>> >>>>> So, what if I fake a PNP, by wrapping, say, a BFT92 around the NPN? >>>>> http://seventransistorlabs.com/Images/Sziklai_Cascode.png >>>>> That'd be Q1 = BFT92, Q2 = 2SC4821, Q3 = 2SA1220A, and resistors for >>>>> flavor, but probably roughly representative. (Ground wouldn't actually be >>>>> ground-ground, but probably something like +40V, and "+12V" would be >>>>> +45V.) >>>>> >>>>> The combination is still fast... ah, but Sziklai connections have a >>>>> propensity for oscillation all their own, let alone in a cascode, plus >>>>> whatever other machinations I might have for feedback around the thing. >>>>> Game killer? >>>>> >>>>> The other option would be folded cascode, which is understandably rather >>>>> wasteful for a power stage! >>>>> >>>>> Tim >>>> >>>> What's the frequency and the bandwidth requirement? >>>> >>>> Regular low-frequency circuit topologies generally don't scale well >>>> into RF amps. >>>> >>> >>> >>> Does anyone use that admittance/Z parameter matrix stuff for RF >>> amp design/stability analysis that I've read about in books like >>> "Intro to RF Design" by Wes Hayward, or do they just mess with it >>> in Spice and are then like "eh works well enough ship >>> it" >>> >>> It seems difficult to even find device data sheets that have those >>> parameters evaluated at enough data points to be useful (so if >>> you don't have a 20k network analyzer to check it yourself at the >>> frequency of interest you're SOL), and I once searched for >>> software that could deal with such things that didn't cost 5k and >>> I couldn't really find much. >>> >> >> The classical approach has the great virtue that it tells you how well >> you _can_ do, so you don't wind up attempting recreational >> impossibilities on the one hand, or turning a silk purse back into a >> sow's ear on the other. >> >> Cheers >> >> Phil "Do the math" Hobbs >> > > That approach cetainly is appealing, but it seems like there often > just aren't the tools available to do it. > > Do I have to do all the impedande/admittance matrix inversions > myself, or write my own software? If the required data is even > available?It isn't difficult--probably 50 lines of Matlab that you can use over and over. Suck it up.> > This is 2k16, come on! I have Angry Birds waiting on me...You've already said that you're a moral relativist, so there's no use appealing to your better nature. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
