If you mean the first amplifier after the antenna, then a reasonable gain would be 20 dB.

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RadioRon

 

Hi, is the BJT amplifier suitable for a sin wave with a swing around 4vpp amplification? Or any suggestion on the amplifier that should i build for the above-mentioned purpose?

By the way, there is only one amplifier for each channel in my design (as shown in the diagram). Will the amplifier provide enough amplification (20dB) for the received signal? Or should i add another amplifier in the design (each channel in receiver part)?

I have designed an amplifier using 2N2222, however the gain is just around 10... Is there anything that i have done wrongly in designing this amplifier?

Are 2N2222 and 2N2222A the same BJT transistor?

Attached Images

block.jpg (153.7 KB, 5 views)

Attached Files

amp.pdf (37.2 KB, 5 views)

 

I do not understand. You have not defined the purpose of this amplifier enough yet. Is this amplifier shown in your diagram? If you are referring to the RF amplifier between the filters on your diagram, then I do not expect the signal to get as high as 4Vpp. This is meant to be, as we call it, a small signal amplifier, that is, one that does not have to deal with large voltage swings.

For each channel, we have the choice of having only 20 dB to start with, using one transistor, or we can decide to put two transistor amplifiers in cascade to get 40 dB. More than this would not be a good idea. The sensitivity of the phase detector is about -60 dBm (if I recall correctly), so we can expect that one amplifier stage in front of that chip will improve the sensitivity to about -75 dBm. This might be enough for your purpose. I suggest that we settle for using one amplifier for each of the two channels in hopes of getting sensitivity of -75 dBm. It is important that the amplifier provide not only power gain, but also a reasonable noise figure. For now, we will focus on gain and not worry too much about noise figure since this is difficult to measure anyway. We might assume that if we build a good amplifier, we might hope for a noise figure of about 4 dB or less.


Your design is not good for high frequency use. I suggest that you change it to become a tuned RF amplifier. Instead of using a resistor load on the collector, you should use a tuned circuit (inductor and capacitor in parallel) resonating at 1 MHz, and then tap the inductor about 20% down from the DC power supply to feed the next stage. Here are some examples of tuned rf amplifiers:

http://www.tpub.com/neets/book8/31g.htm

Usually, the collector uses a tank circuit instead of a resistor, but a tuned circuit is often also used at the base. You don't need the tuned circuit on the base, but the one on the collector is essential to getting good performance.

these links may give more useful example info:

http://www.isp.ca/ve3nh/w7iuv.htm
http://www.isp.ca/ve3nh/w1fb.htm
http://freespace.virgin.net/geoff.co...3PQA-amp-1.jpg
http://www.n3ox.net/projects/rxloop/

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RadioRon

 

Sorry, I have misconceived the usage of small signal amplifier. By the way, i am planning to add a power amplifier to the output of the crystal oscillator to boost the transmitting power of the transmitter to 20dBm (100mW) if possible. Do you have any suggestion on the type of amplifier that i can build?


I have rebuild the small signal amplifier, and it works quite well where it produces 20*log (37/2) = approximately 25dB of gain. I can only simulate the circuit using the signal ranging from 1micro to 1nano Vpp. The simulator went wrong when I was trying to simulate the circuit using 1 pico Vpp signal. On your opinion, does this amplifier work well? (The diagram and waveform included).


Did i connect the tank filter to the amplifier correctly? By the way, I cant get the output signal from the amplifier... (Please refer to tank amp.pdf)

Attached Images

nano.JPG (161.6 KB, 2 views)

Attached Files

	25dB Amp.pdf (39.4 KB, 1 views)
	tank amp.pdf (40.8 KB, 2 views)

 

Well, I have finished the RF amplifier design, it seems working well (with a gain of approximately 30dB). It seems the tank filter greatly improve the gain of the amplifier... What do you think about this amplifier?

However, there is a problem where the output of the amplifier for the signal with nano volt range is not desirable as the output waveform (the waveform is perfect) is shifted up and down in simulation. It seems its due to input coupling capacitor as I try varying the value of that capacitor and investigate the result waveform for the 1uV-range input, the waveform shifted up and down (similar to the above-mentioned situation) when i adjust the capacitor with the value smaller than it supposed to be... What do you think?

Attached Files

RF Amplifier.pdf (41.6 KB, 5 views)

 

I don't know why the simulation is behaving that way. The input capacitor is normally chosen to have a low impedance compared to the source and load resistances that it is feeding. This implies that the larger the capacitance,the better, but in practice we find that when the capacitance is too big, it begins to have too much series inductance which is undesireable. So, we usually choose something smaller. For example, if we want the capacitor to have an impedance less than 1 ohm then it would have to be 160 nF or larger. Your choice of 100 nF seems ok.

I'm not happy with your choice of base bias resistors. The ratio of base resistors to the emitter resistor is simply too large and cannot be supported by a current gain of only 100 such as the 2n222 might have. You need to either increase the emitter resistor, which will reduce the total collector current, or you have to reduce the base bias resistors so that the base current necessary to drive the emitter resistor doesn't cause a significant voltage drop across the upper base bias resistor.

While the tank circuit topology seems OK, I'm surprised that the tap ratio is so high. You have tapped the 50 ohm load about 90% across the tank circuit, which dramatically reduces the tank's loaded Q. These types of circuits usually work better when the loaded Q is about 10 to 30% less than the unloaded Q.

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RadioRon

 

I have redesigned the circuitry to lower the resistance of the voltage divider. I tried to tap the 50 ohm load about 10% across the tank circuit ( i mean i tap it with the tapped value in the previous circuitry up side down), the gain was however greatly decreased (to 0.18dB...), and the waveform is shifted up and down... I wonder why this happens...

By the way, i am planning to add a power amplifier to the output of the crystal oscillator to boost the transmitting power of the transmitter to 20dBm (100mW) if possible. The oscillator output swings around 4 volts... Do you have any suggestion on the type of amplifier that i can build?

Attached Files

RFAmp30dB.pdf (41.7 KB, 5 views)

 

Oh, I see why that happens. You have the 50 ohm load DC coupled to the tank circuit which messes up the bias very badly. You need to put a 100 nF cap in series with the output load to block DC.

There are many types of amps you can use. I will consider and repost

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RadioRon

 

I recommend that you use the same amplifier topology as you already have using the 2n2222 but adjust the emitter resistor so that the collector current remains relatively high so that the amplifier does not limit until it can deliver about 2.5 Volts RMS into the 50 ohm output. You may have to adjust the tapping ratio and your base bias, but it should be able to give you the power output you want.

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RadioRon

 

There is no gain improvement and output improvement even the 100 nF cap is put in series with the output load, unless the load is tapped at 90% across the tank circuit...

 

I don't see any coupling between L1 and L2 on the schematic. Is this correct?

 

Perhaps you have indeed found the correct impedance match point then.

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RadioRon

 

Well, this is the updated schematic of the circuitry... The amplifier seems working very well, the output is desirable for the pico volt input...

As I know, the hfe value for individual transistor might differ even they are same model. Should I measure the hfe value of the transistor and rearrange the circuitry to match the hfe value?

Regarding the phase detector, I cant perform the simulation for this circuit as there is no pspice model for this component: AD8302. Do you have any suggestion on the way that i should determine the values for all the components associated with the circuitry? Or should I refer to the circuitry in data sheets here (AD8302.pdf and Operation of RF Detector Products at Low Frequency.pdf)?

Attached Files

	RFAmp30dB.pdf (42.3 KB, 2 views)
	AD8302.pdf (563.1 KB, 1 views)
	Operation of RF Detector Products at Low Frequency.pdf (779.0 KB, 1 views)

 

Your bias design is the type that insures that HFE variation does not significantly affect the operating point, so you don't need to check each transistor.

It is common practice to use the application circuit suggested in the data sheet or application notes from the manufacturer. This is especially true for ICs with complex functions such as this one.

I find it a bit humorous that you say the amplifier works well. Of course I understand that you mean in simulation only. In my experience, something that works well in simulation does not necessarily work well in practice, especially at RF frequencies. The simulation does not account for stray coupling, non-ideal characteristics of components, stray inductance and capacitance, high impedance grounds, magnetic 120Hz induced noise, power supply noise, AM broadcast radio interference, changes in component values due to heating, bad soldering, undesired feedback, and a few other things. The simulation is extremely useful to get the basics worked out though, so it is still a valuable step in design.

__________________
RadioRon

 

Well, what should I do next? Should I try implementing this amplifier circuitry on the PCB to see the result? By the way, it seems so complicated to have transistor in amplifier design. Is there other option for me to build this amplifier? Besides, I have limited budget on this project...