You mean like using transistors for Rload? I am actually using resistors of 500 ohm. Is it better to use transistors for that?
No, no. Perhaps I didn't explained my self clearly. Your load must be a resistor, no doubt about it. I was refering to the top transistor's. Like in a simple amplifier, your gain is defined by Gain=-gm*Ro. The Ro can either be a resistor or a transistor. That's what I call a load transistor. What are you using as Ro,resistors or transistors?
Post an image of your circuit so I can understand better. Besides that, I wanna know if it's a balanced mixer or not.
I am not sure what you mean by buffering. can you explain, please?
If your using load transistors (be carefull, no confusion) your output impedance will probably be very high (several kOhms, which is good). If you connect your circuit to a load (like 500 Ω resistor, antenna, or whatever) your gain will drop dramatically. That problem is usually solved connecting the output of your mixer to a buffer, like a common drain amplifier, which as voltage gain = 1 but very low output impedance (around 50 Ω, which is good)
Well, the RF signal is 2.4 GHz and the LO signal is 1.8 GHz. This is what I use for testing purposes.In the real bluetooth application it would probably be something like 2.4 GHZ RF and 2.2 GHz LO. I guess 200MHz is about right for the IF signal in the bluetooth receiver application. So I don't think bandwidth is the bottleneck factor here.
You're working with very high frequencies. This means you cannot use big transistors because they will reduce your bandwidth. You should plot 2 AC simulations, 1 applying only the LO signal and the other applying only the RF signal, and check the output bandwidth. If the cut-off frequency is bellow 2.4 GHz then you have a problem. If so you'll have to reduce all the transistor's sizes to reazonable values.
Let me tell a little bit of personal experience. Once I tried to build a balanced Gilbert mixer for 5 GHz using 0.38um technology, when I was still a noob. It failed miserably. After a while I discovered why: for the 0.38 technology, even the simplest inverter circuit with the smaller transistors had a maximum cut-off frequency of about 4 GHz (insufficient). The whole mixer circuit had an even worst frequency performance.
So, moral of the story, check the bandwidth first. It's useless to try improving your gain if your bandwidth is not enough.