Hi all,

Recently I met an interesting question for RF amplifier design. In the design, at a frequency of about 2 GHz (e.g. 2.4 GHz), we want to design an amplifier which can independently amplify "current" or independently amplify "voltage".

At low frequency, we know that amplification circuits can be classified into three categories: "power" amplification circuit, "current" amplification circuit, and "voltage" amplification circuit. For example, a BJT Common-Emitter (CE) amplifier can amplify current and voltage simultaneously. A Common-Collector (CC) amplifier mainly amplify current (i.e. amplifier voltage gain is about 1). A Common-Base (CB) amplifier mainly amplify voltage (i.e. amplifier output current almost has no change).   But at RF frequency, typically at about 2 GHz, I cannot find such amplification circuits which can independently (or mainly) amplify the "current" or "voltage".

At about 2 GHz, are there such current amplification circuits which can mainly amplify the current (with small small voltage gain), or voltage amplification circuits which can mainly amplify the voltage (with small small current gain) ?

Doesn't anyone have an idea or any comments? It is an imperative duty! Thanks!

I agree with rfidea. The technologies now are so fast that a lot of your low frequency designs can work into the low GHz range. It will really depend on what kind of power consumption and NF you want to achieve.


cheers,
Aaron

Thank you so much for your reply. Those days, I try to construct a BJT amplifier base on Common-Collector (CC) and Common-Base (CB), respectively. The BJT chosen for this design is AT41411, which has a cut-off frequency (ft) of 7 GHz. But, the ADS simulation results are strange. Take the CC amplifier for example, the emitter current (IE) is bigger than the base current (IB), but the output current (I_out) becomes lower than the input current (I_in). According to the characteristics of a BJT CC amplifier, the output current should be amplified. I don’t know why. (Of course, what is correct is that the output voltage becomes a litter lower than the input voltage, because the amplifier gain is between 0 and 1)

The simulation circuit and results are attached for your review. Looking forward for your reply. Best regards!

Hi, rfidea. Thank you for your reply. The idea of using CC and CB to as a current and gain device, respectively, is from the low-frequency analogy circuit. To be honest, I am not sure whether it can work or not at RF. The problem of a CE (or CG) amplifier is that the current and voltage will be amplified simultaneously, and it is not what I want. For a CE (or CG) amplifier, do you know whether it can mainly amplify current or voltage, while has low gain for the other one.

Could you suggest a BJT device (and a FET device) ?  If I want my amplification circuit to work at 2GHz, whether the device’s maxFt should be higher than 25GHz as you suggested first ? I am sorry I don’t have much experience in RF circuit design. Cloud you give me a hand ? Thanks and Best Regards!

jianke wrote on Mar 1^st, 2011, 12:40pm:

A suggestion -

Please tell us what you are trying to do, and perhaps we can offer suggestions.

Not the circuit you are trying to create, but rather what you are trying to do. (example, you have a 2.5GHz signal with XX amplitude, coming from a 50 ohm cable and you need to amplify it to XX amplitude, and feed it into another cable at 50 ohms, you need a linearity of IP3 = XXdb, and your Noise Figure has to be XX)

At these frequencies, you have to know what you are doing and parasitics and many other factors will effect what you are doing.

At the frequency mentioned op-amps are generally not used.

If you can't define what you need to do, its not going to be easy to help.

Some clarification, because of a direct question from jianke, about the suggestions of using technologies with maxFt > 25GHz for this problem at 2GHz.

I did not mean that there is a strict requirement that the technology used must be higher than 25GHz. I only mean what I said, that there is a lot of technologies available with maxFt > 25GHz that can be used.

Ft is the frequency where the transitor has unity gain. If that frequency is higher than your operating frequency it is possiblilty to achive gain in your circuit.