# What's the purpose of this section in an RF amplifier circuit?

I'm relatively new to electronics and I've just grasped the concept of bypass caps and some common decoupling practices (so you have an idea of my general level) – but I can't seem to understand the purpose of the circled section of this circuit:

As I said, I'm very new to electronics so don't be too hard on me – it's probably something blatantly obvious I'm missing. Or not.

• Where did the circuit come from and what operating RF frequency does it use. Also, L2 seems to be strangely specified. – Andy aka Dec 15 '15 at 12:06
• R2, R3, R4together provide DC bias. The spec on L2 MAY mean eg 6 turns wide x 3 layers deep on a 0.4 some-units former - or something else :-). Without C3 I'd have guessed it was simply an RF choke to prevent RF feedback BUT as C3 bypasses R2 and as R2 is >> R3 it suggests that it is intended to allow RF feedback controlled by R3 & L2. It's negative feedback so would stabilise the gain. Arguably R3 L2 have a frequency dependant effect with impedance increasing with frequency but it's barn-door wide. – Russell McMahon Dec 15 '15 at 12:12
• @Andyaka the tiny yellow box says 10-500 Mhz when I squint and peer. – Ecnerwal Dec 15 '15 at 12:23
• I'm thinking L2 is a stripline inductor but the numbers don't seem to give it much inductance (length 6, width 3, thickness 0.4mm means 2.3 nH) – Andy aka Dec 15 '15 at 12:57

This appears to be your source, or a source - Wideband RF amplifier

Their explanation is close to my one in the comments above. I noted that the R2 R3 L2 path provides negative feedback with impedance rising with frequency - which decreases feedback and so increases gain, as they note.

It says

This is a classical RF amplifier design.
The feedback resistors R3 and R2 define the gain.
By changing the values we can get a higher gain at low frequencies, but this will influence the overall gain taper of the amplifier.
Coil L2 is used to get more gain at high frequencies.
If the value is increased, instability will occur.

My above L2 guess was only partially correct. They say:

Dimensions of L2 are: 6 windings, inner diameter is 3mm and wire thickness must be 0.4mm. In the proto type I used and SMD type for L1, make sure it’s a high Q coil.

My original assessment:

R2, R3, R4 together provide DC bias.
The spec on L2 MAY mean eg 6 turns wide x 3 layers deep on a 0.4 some-units former - or something else :-).

Without C3 I'd have guessed it was simply an RF choke to prevent RF feedback BUT as C3 bypasses R2 and as R2 is >> R3 it suggests that it is intended to allow RF feedback controlled by R3 & L2.

It's negative feedback so would stabilise the gain.
Arguably R3 L2 have a frequency dependant effect with impedance increasing with frequency but it's barn-door wide.

• Yes, that's the source – found it at random and it just puzzled me. So, just to see if I understand this correctly: this is essentially a feedback loop designed to boost the overall gain of the amplifier, with L2 placed where it is to favor higher frequencies over lower ones – and the whole circuit can be calibrated by changing the values of L2 and R3 respectively. This may sound stupid, but my first impression would be that L2's purpose was to reduce the gain over higher frequencies (higher impedance) – not boost them. How does that work? – Fryxell Dec 15 '15 at 13:39
• @Fryxell You have it right. | As above " ... It's negative feedback so would stabilise the gain. ..." ie negative feedback reduces gain BUT " ... with impedance increasing with frequency ..." so that gain rises with frequency. Which matches his " ... Coil L2 is used to get more gain at high frequencies. ...". – Russell McMahon Dec 16 '15 at 14:02

R2/3/4 together with R1/5 provide the DC bias. The collector current though R1 and the emitter current through R5 stabilise the bias.

C3 shorts R2 at RF, so that R3 can control the low frequency gain.

L2 increases the impedance of the signal feedback path as the frequency rises. This is a quite common method to 'lift' the gain at higher frequencies. Transistors have relatively low gain, and some parasitic capacitance, so the R3 feedback resistor has only limited effect in controlling the gain, and the close loop gain is obviously running out of steam at the top end of the range. L2 also has the effect of tuning out some of the transistor C at high frequencies, though this effect is quite small, nowhere near resonant, due to the de-Qing effect of R3 on L2.