class AB amp self oscillating

[WA1GFZ]

each stage has feedback My suggestion just increased the FB at higher frequencies to tame it. Good power supply bypass is always a good idea

[KM1H]

km1h, what is the advantage of a 2n5109 over a 2n3866? the only differences i see are that 2n5109 is rated for 1w of power dissipation whereas the 2n3866 is rated at 5w and the 2n5109 seems to have a higher minimum hfe. anything else? would i be able to get a higher gain with a 2n5109?

The 3866 is rated at 1W out in Class C while the 5109 designed as a Class A CATV amp wil double that in Class C and have about 1.5dB more gain. Ive also found it easier to stabilize even tho its Gbw is higher

carl (km1h), would a ferrite bead on the base potentially eliminate the feedback without limiting gain at higher frequency (unlike a feedback capacitor.

That depends upon what you mean by higher frequency. If you use the minimum ferrite to eliminate a VHF parasitic then it should have no loss effect at HF. One tiny FB43-101 may be all it takes. They also work on the other leads.

Carl

[ssbothwell KJ6RSG]

WA1GFZ, i read through AN749 but i am still unclear about how to determine the input Z circuit. i'm not really sure what i am reading in this article. it seems to be tables of impedance values for various transformer setups. how does that help me know what impedance i need to match?

[WA1GFZ]

Simple, the load is 50 ohms on the output.
So if you have a 1:2 transformer the output stage needs to drive 12.5 ohms
Push pull each side drives 6.25 ohms.
Now if you have a 1:3 the output stage needs to drive 5.55 ohms or each side of the push pull 2.7 ohms.
A 1:4 transformer loads the final stage to 3.125 ohms so each side drives 1.56 ohms.
Look at your output stage and select the transformer that matches the power
rating of the output device.
Typical 12 volt 100 watt rig runs 1:4 or 1:5 output transformer.
A pair of 48 volt devices will make 300 watts with a 1:2 output transformer.
Go to CCI site and download a bunch of old motorola applications to get ideas. Helge wrote the bible on this stuff back in the 70s.

[ssbothwell KJ6RSG]

i arbitrarily chose a 1:2 ratio and used this calculator to generate the pi-network: http://www.qsl.net/wa2whv/radiocalcs.shtml

i entered:
input Z = 12.5
output Z = 50
Q = 5
freq = 7MHz

and it gave me the part list:
C1 = 2752pF
L1 = .41uH
C2 =1585pF

i added the pi-network to the amp output and it produces a clean sine wave. however it the amp has signifigantly less gain. without the pi-network it was producing about +26dB gain and with the pi-network it produces +13dB @ 7MHz. the gain is very frequency dependent with it producing +20dB at 6.4MHz.

would a different impedance ratio in the pi-network produce greater gain?

[EDITED]replaced all mW and mV measurements with dBm and +dB gain to clarify this post.

attached images are the circuit set to ~6.4MHz and 7MHz.

[WA1GFZ]

yes, Your pi network is transforming the impedance
As long as the output device can handle the extra load.

[ssbothwell KJ6RSG]

messing around in LTSpice i tried a bunch of Z ratios. i highest peak voltage i was able to get was 4.1V with 25ohm inputZ and 50ohm outputZ ratio. the lower i set the input Z, the lower my peak voltage. for example 5.55ohm input Z yields 3.5V peak output.

does this make sense and seem right?

is the basic process here to test out different Z ratios until you get the circuit to peak?

edit: is it possible to have a pi-network that can be adjusted to work across a variety of frequency bands?

[WA1GFZ]

Solid state rigs have a fixed low pass filter for each band. You may have to run class A to get a clean signal through a fixed filter. The output transformer ratio would be 2:1 broadband. this will force 12.5 ohms on the collector and 50 ohms to load/filter. This is a very common configuration for the 3866.
Class AB will produce some distortion when the waveform clips.